遇到多个构造器参数考虑用构建器(Builder)


如果一个对象带有多个参数,程序员一般采用重叠构造器模式。但是该方法的缺点是,如果参数很长,客户端代码不仅很难编写,同时可读性也很差。

第二种替代方案:Java Beans。类中提供一个无参构造方法和对属性的mutator方法。该方法的缺点是容易线程安全问题。

第三种方案就是Builder模式。该方案既保证了重叠构造器模式的安全性,又保证像JavaBeans模式那样好的可读性。

考虑一个例子,一款食物产品通常附属有营养标识,有必须的,也有可选的。比如脂肪,钠,碳水化合物,加路里等。

下面用Builder模式来构造一个营养标识类(NutritionFacts):


// Builder Pattern
public class NutritionFacts {

	private final int servingSize;
	private final int servings;
	private final int calories;
	private final int fat;
	private final int sodium;
	private final int carbohydrate;

	// nested builder
	public static class Builder {
		// Required parameters
		private final int servingSize;
		private final int servings;

		// optional parameters - initialized to default values

		private int calories = 0;
		private int fat = 0;
		private int carbohydrate = 0;
		private int sodium = 0;

		public Builder(int servingSize, int servings) {
			this.servingSize = servingSize;
			this.servings = servings;
		}

		public Builder calories(int val) {
			calories = val;
			return this;
		}

		public Builder fat(int val) {
			fat = val;
			return this;
		}

		public Builder carbohydrate(int val) {
			carbohydrate = val;
			return this;
		}

		public Builder sodium(int val) {
			sodium = val;
			return this;
		}

		public NutritionFacts build() {
			return new NutritionFacts(this);
		}
	}

	private NutritionFacts(Builder builder) {
		servingSize = builder.servingSize;
		servings = builder.servings;
		calories = builder.calories;
		fat = builder.fat;
		sodium = builder.sodium;
		carbohydrate = builder.carbohydrate;
	}

	@Override
	public String toString() {
		return "NutrionFacts[servingSize:" + servingSize + ",servings:"
				+ servings + ",fat:" + fat + ",calories:" + calories
				+ ",sodium:" + sodium + ",carbohydrate:" + carbohydrate + "]";
	}

	public static void main(String args[]) {

		NutritionFacts nf = new NutritionFacts.Builder(5, 10).calories(1)
				.fat(2).carbohydrate(3).build();	// by default sodium = 0

		System.out.println(nf);
	}

}

模式在Android SDK中也有应用。比如我们最熟悉的警告框-AlertDialog,该类就嵌套了一个共有的静态Builder类。如下:

 public static class Builder {
        private final AlertController.AlertParams P;
        private int mTheme;
        
        /**
         * Constructor using a context for this builder and the {@link AlertDialog} it creates.
         */
        public Builder(Context context) {
            this(context, com.android.internal.R.style.Theme_Dialog_Alert);
        }

        /**
         * Constructor using a context and theme for this builder and
         * the {@link AlertDialog} it creates.
         */
        public Builder(Context context, int theme) {
            P = new AlertController.AlertParams(context);
            mTheme = theme;
        }
        
        /**
         * Set the title using the given resource id.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setTitle(int titleId) {
            P.mTitle = P.mContext.getText(titleId);
            return this;
        }
        
        /**
         * Set the title displayed in the {@link Dialog}.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setTitle(CharSequence title) {
            P.mTitle = title;
            return this;
        }
        
        /**
         * Set the title using the custom view {@code customTitleView}. The
         * methods {@link #setTitle(int)} and {@link #setIcon(int)} should be
         * sufficient for most titles, but this is provided if the title needs
         * more customization. Using this will replace the title and icon set
         * via the other methods.
         * 
         * @param customTitleView The custom view to use as the title.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setCustomTitle(View customTitleView) {
            P.mCustomTitleView = customTitleView;
            return this;
        }
        
        /**
         * Set the message to display using the given resource id.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setMessage(int messageId) {
            P.mMessage = P.mContext.getText(messageId);
            return this;
        }
        
        /**
         * Set the message to display.
          *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setMessage(CharSequence message) {
            P.mMessage = message;
            return this;
        }
        
        /**
         * Set the resource id of the {@link Drawable} to be used in the title.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setIcon(int iconId) {
            P.mIconId = iconId;
            return this;
        }
        
        /**
         * Set the {@link Drawable} to be used in the title.
          *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setIcon(Drawable icon) {
            P.mIcon = icon;
            return this;
        }
        
        /**
         * Set a listener to be invoked when the positive button of the dialog is pressed.
         * @param textId The resource id of the text to display in the positive button
         * @param listener The {@link DialogInterface.OnClickListener} to use.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setPositiveButton(int textId, final OnClickListener listener) {
            P.mPositiveButtonText = P.mContext.getText(textId);
            P.mPositiveButtonListener = listener;
            return this;
        }
        
        /**
         * Set a listener to be invoked when the positive button of the dialog is pressed.
         * @param text The text to display in the positive button
         * @param listener The {@link DialogInterface.OnClickListener} to use.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setPositiveButton(CharSequence text, final OnClickListener listener) {
            P.mPositiveButtonText = text;
            P.mPositiveButtonListener = listener;
            return this;
        }
        
        /**
         * Set a listener to be invoked when the negative button of the dialog is pressed.
         * @param textId The resource id of the text to display in the negative button
         * @param listener The {@link DialogInterface.OnClickListener} to use.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setNegativeButton(int textId, final OnClickListener listener) {
            P.mNegativeButtonText = P.mContext.getText(textId);
            P.mNegativeButtonListener = listener;
            return this;
        }
        
        /**
         * Set a listener to be invoked when the negative button of the dialog is pressed.
         * @param text The text to display in the negative button
         * @param listener The {@link DialogInterface.OnClickListener} to use.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setNegativeButton(CharSequence text, final OnClickListener listener) {
            P.mNegativeButtonText = text;
            P.mNegativeButtonListener = listener;
            return this;
        }
        
        /**
         * Set a listener to be invoked when the neutral button of the dialog is pressed.
         * @param textId The resource id of the text to display in the neutral button
         * @param listener The {@link DialogInterface.OnClickListener} to use.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setNeutralButton(int textId, final OnClickListener listener) {
            P.mNeutralButtonText = P.mContext.getText(textId);
            P.mNeutralButtonListener = listener;
            return this;
        }
        
        /**
         * Set a listener to be invoked when the neutral button of the dialog is pressed.
         * @param text The text to display in the neutral button
         * @param listener The {@link DialogInterface.OnClickListener} to use.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setNeutralButton(CharSequence text, final OnClickListener listener) {
            P.mNeutralButtonText = text;
            P.mNeutralButtonListener = listener;
            return this;
        }
        
        /**
         * Sets whether the dialog is cancelable or not default is true.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setCancelable(boolean cancelable) {
            P.mCancelable = cancelable;
            return this;
        }
        
        /**
         * Sets the callback that will be called if the dialog is canceled.
         * @see #setCancelable(boolean)
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setOnCancelListener(OnCancelListener onCancelListener) {
            P.mOnCancelListener = onCancelListener;
            return this;
        }
        
        /**
         * Sets the callback that will be called if a key is dispatched to the dialog.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setOnKeyListener(OnKeyListener onKeyListener) {
            P.mOnKeyListener = onKeyListener;
            return this;
        }
        
        /**
         * Set a list of items to be displayed in the dialog as the content, you will be notified of the
         * selected item via the supplied listener. This should be an array type i.e. R.array.foo
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setItems(int itemsId, final OnClickListener listener) {
            P.mItems = P.mContext.getResources().getTextArray(itemsId);
            P.mOnClickListener = listener;
            return this;
        }
        
        /**
         * Set a list of items to be displayed in the dialog as the content, you will be notified of the
         * selected item via the supplied listener.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setItems(CharSequence[] items, final OnClickListener listener) {
            P.mItems = items;
            P.mOnClickListener = listener;
            return this;
        }
        
        /**
         * Set a list of items, which are supplied by the given {@link ListAdapter}, to be
         * displayed in the dialog as the content, you will be notified of the
         * selected item via the supplied listener.
         * 
         * @param adapter The {@link ListAdapter} to supply the list of items
         * @param listener The listener that will be called when an item is clicked.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setAdapter(final ListAdapter adapter, final OnClickListener listener) {
            P.mAdapter = adapter;
            P.mOnClickListener = listener;
            return this;
        }
        
        /**
         * Set a list of items, which are supplied by the given {@link Cursor}, to be
         * displayed in the dialog as the content, you will be notified of the
         * selected item via the supplied listener.
         * 
         * @param cursor The {@link Cursor} to supply the list of items
         * @param listener The listener that will be called when an item is clicked.
         * @param labelColumn The column name on the cursor containing the string to display
         *          in the label.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setCursor(final Cursor cursor, final OnClickListener listener,
                String labelColumn) {
            P.mCursor = cursor;
            P.mLabelColumn = labelColumn;
            P.mOnClickListener = listener;
            return this;
        }
        
        /**
         * Set a list of items to be displayed in the dialog as the content,
         * you will be notified of the selected item via the supplied listener.
         * This should be an array type, e.g. R.array.foo. The list will have
         * a check mark displayed to the right of the text for each checked
         * item. Clicking on an item in the list will not dismiss the dialog.
         * Clicking on a button will dismiss the dialog.
         * 
         * @param itemsId the resource id of an array i.e. R.array.foo
         * @param checkedItems specifies which items are checked. It should be null in which case no
         *        items are checked. If non null it must be exactly the same length as the array of
         *        items.
         * @param listener notified when an item on the list is clicked. The dialog will not be
         *        dismissed when an item is clicked. It will only be dismissed if clicked on a
         *        button, if no buttons are supplied it's up to the user to dismiss the dialog.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setMultiChoiceItems(int itemsId, boolean[] checkedItems, 
                final OnMultiChoiceClickListener listener) {
            P.mItems = P.mContext.getResources().getTextArray(itemsId);
            P.mOnCheckboxClickListener = listener;
            P.mCheckedItems = checkedItems;
            P.mIsMultiChoice = true;
            return this;
        }
        
        /**
         * Set a list of items to be displayed in the dialog as the content,
         * you will be notified of the selected item via the supplied listener.
         * The list will have a check mark displayed to the right of the text
         * for each checked item. Clicking on an item in the list will not
         * dismiss the dialog. Clicking on a button will dismiss the dialog.
         * 
         * @param items the text of the items to be displayed in the list.
         * @param checkedItems specifies which items are checked. It should be null in which case no
         *        items are checked. If non null it must be exactly the same length as the array of
         *        items.
         * @param listener notified when an item on the list is clicked. The dialog will not be
         *        dismissed when an item is clicked. It will only be dismissed if clicked on a
         *        button, if no buttons are supplied it's up to the user to dismiss the dialog.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setMultiChoiceItems(CharSequence[] items, boolean[] checkedItems, 
                final OnMultiChoiceClickListener listener) {
            P.mItems = items;
            P.mOnCheckboxClickListener = listener;
            P.mCheckedItems = checkedItems;
            P.mIsMultiChoice = true;
            return this;
        }
        
        /**
         * Set a list of items to be displayed in the dialog as the content,
         * you will be notified of the selected item via the supplied listener.
         * The list will have a check mark displayed to the right of the text
         * for each checked item. Clicking on an item in the list will not
         * dismiss the dialog. Clicking on a button will dismiss the dialog.
         * 
         * @param cursor the cursor used to provide the items.
         * @param isCheckedColumn specifies the column name on the cursor to use to determine
         *        whether a checkbox is checked or not. It must return an integer value where 1
         *        means checked and 0 means unchecked.
         * @param labelColumn The column name on the cursor containing the string to display in the
         *        label.
         * @param listener notified when an item on the list is clicked. The dialog will not be
         *        dismissed when an item is clicked. It will only be dismissed if clicked on a
         *        button, if no buttons are supplied it's up to the user to dismiss the dialog.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setMultiChoiceItems(Cursor cursor, String isCheckedColumn, String labelColumn, 
                final OnMultiChoiceClickListener listener) {
            P.mCursor = cursor;
            P.mOnCheckboxClickListener = listener;
            P.mIsCheckedColumn = isCheckedColumn;
            P.mLabelColumn = labelColumn;
            P.mIsMultiChoice = true;
            return this;
        }
        
        /**
         * Set a list of items to be displayed in the dialog as the content, you will be notified of
         * the selected item via the supplied listener. This should be an array type i.e.
         * R.array.foo The list will have a check mark displayed to the right of the text for the
         * checked item. Clicking on an item in the list will not dismiss the dialog. Clicking on a
         * button will dismiss the dialog.
         * 
         * @param itemsId the resource id of an array i.e. R.array.foo
         * @param checkedItem specifies which item is checked. If -1 no items are checked.
         * @param listener notified when an item on the list is clicked. The dialog will not be
         *        dismissed when an item is clicked. It will only be dismissed if clicked on a
         *        button, if no buttons are supplied it's up to the user to dismiss the dialog.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setSingleChoiceItems(int itemsId, int checkedItem, 
                final OnClickListener listener) {
            P.mItems = P.mContext.getResources().getTextArray(itemsId);
            P.mOnClickListener = listener;
            P.mCheckedItem = checkedItem;
            P.mIsSingleChoice = true;
            return this;
        }
        
        /**
         * Set a list of items to be displayed in the dialog as the content, you will be notified of
         * the selected item via the supplied listener. The list will have a check mark displayed to
         * the right of the text for the checked item. Clicking on an item in the list will not
         * dismiss the dialog. Clicking on a button will dismiss the dialog.
         * 
         * @param cursor the cursor to retrieve the items from.
         * @param checkedItem specifies which item is checked. If -1 no items are checked.
         * @param labelColumn The column name on the cursor containing the string to display in the
         *        label.
         * @param listener notified when an item on the list is clicked. The dialog will not be
         *        dismissed when an item is clicked. It will only be dismissed if clicked on a
         *        button, if no buttons are supplied it's up to the user to dismiss the dialog.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setSingleChoiceItems(Cursor cursor, int checkedItem, String labelColumn, 
                final OnClickListener listener) {
            P.mCursor = cursor;
            P.mOnClickListener = listener;
            P.mCheckedItem = checkedItem;
            P.mLabelColumn = labelColumn;
            P.mIsSingleChoice = true;
            return this;
        }
        
        /**
         * Set a list of items to be displayed in the dialog as the content, you will be notified of
         * the selected item via the supplied listener. The list will have a check mark displayed to
         * the right of the text for the checked item. Clicking on an item in the list will not
         * dismiss the dialog. Clicking on a button will dismiss the dialog.
         * 
         * @param items the items to be displayed.
         * @param checkedItem specifies which item is checked. If -1 no items are checked.
         * @param listener notified when an item on the list is clicked. The dialog will not be
         *        dismissed when an item is clicked. It will only be dismissed if clicked on a
         *        button, if no buttons are supplied it's up to the user to dismiss the dialog.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setSingleChoiceItems(CharSequence[] items, int checkedItem, final OnClickListener listener) {
            P.mItems = items;
            P.mOnClickListener = listener;
            P.mCheckedItem = checkedItem;
            P.mIsSingleChoice = true;
            return this;
        } 
        
        /**
         * Set a list of items to be displayed in the dialog as the content, you will be notified of
         * the selected item via the supplied listener. The list will have a check mark displayed to
         * the right of the text for the checked item. Clicking on an item in the list will not
         * dismiss the dialog. Clicking on a button will dismiss the dialog.
         * 
         * @param adapter The {@link ListAdapter} to supply the list of items
         * @param checkedItem specifies which item is checked. If -1 no items are checked.
         * @param listener notified when an item on the list is clicked. The dialog will not be
         *        dismissed when an item is clicked. It will only be dismissed if clicked on a
         *        button, if no buttons are supplied it's up to the user to dismiss the dialog.
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setSingleChoiceItems(ListAdapter adapter, int checkedItem, final OnClickListener listener) {
            P.mAdapter = adapter;
            P.mOnClickListener = listener;
            P.mCheckedItem = checkedItem;
            P.mIsSingleChoice = true;
            return this;
        }
        
        /**
         * Sets a listener to be invoked when an item in the list is selected.
         * 
         * @param listener The listener to be invoked.
         * @see AdapterView#setOnItemSelectedListener(android.widget.AdapterView.OnItemSelectedListener)
         *
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setOnItemSelectedListener(final AdapterView.OnItemSelectedListener listener) {
            P.mOnItemSelectedListener = listener;
            return this;
        }
        
        /**
         * Set a custom view to be the contents of the Dialog. If the supplied view is an instance
         * of a {@link ListView} the light background will be used.
         *
         * @param view The view to use as the contents of the Dialog.
         * 
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setView(View view) {
            P.mView = view;
            P.mViewSpacingSpecified = false;
            return this;
        }
        
        /**
         * Set a custom view to be the contents of the Dialog, specifying the
         * spacing to appear around that view. If the supplied view is an
         * instance of a {@link ListView} the light background will be used.
         * 
         * @param view The view to use as the contents of the Dialog.
         * @param viewSpacingLeft Spacing between the left edge of the view and
         *        the dialog frame
         * @param viewSpacingTop Spacing between the top edge of the view and
         *        the dialog frame
         * @param viewSpacingRight Spacing between the right edge of the view
         *        and the dialog frame
         * @param viewSpacingBottom Spacing between the bottom edge of the view
         *        and the dialog frame
         * @return This Builder object to allow for chaining of calls to set
         *         methods
         *         
         * 
         * This is currently hidden because it seems like people should just
         * be able to put padding around the view.
         * @hide
         */
        public Builder setView(View view, int viewSpacingLeft, int viewSpacingTop,
                int viewSpacingRight, int viewSpacingBottom) {
            P.mView = view;
            P.mViewSpacingSpecified = true;
            P.mViewSpacingLeft = viewSpacingLeft;
            P.mViewSpacingTop = viewSpacingTop;
            P.mViewSpacingRight = viewSpacingRight;
            P.mViewSpacingBottom = viewSpacingBottom;
            return this;
        }
        
        /**
         * Sets the Dialog to use the inverse background, regardless of what the
         * contents is.
         * 
         * @param useInverseBackground Whether to use the inverse background
         * 
         * @return This Builder object to allow for chaining of calls to set methods
         */
        public Builder setInverseBackgroundForced(boolean useInverseBackground) {
            P.mForceInverseBackground = useInverseBackground;
            return this;
        }

        /**
         * @hide
         */
        public Builder setRecycleOnMeasureEnabled(boolean enabled) {
            P.mRecycleOnMeasure = enabled;
            return this;
        }


        /**
         * Creates a {@link AlertDialog} with the arguments supplied to this builder. It does not
         * {@link Dialog#show()} the dialog. This allows the user to do any extra processing
         * before displaying the dialog. Use {@link #show()} if you don't have any other processing
         * to do and want this to be created and displayed.
         */
        public AlertDialog create() {
            final AlertDialog dialog = new AlertDialog(P.mContext, mTheme);
            P.apply(dialog.mAlert);
            dialog.setCancelable(P.mCancelable);
            dialog.setOnCancelListener(P.mOnCancelListener);
            if (P.mOnKeyListener != null) {
                dialog.setOnKeyListener(P.mOnKeyListener);
            }
            return dialog;
        }

        /**
         * Creates a {@link AlertDialog} with the arguments supplied to this builder and
         * {@link Dialog#show()}'s the dialog.
         */
        public AlertDialog show() {
            AlertDialog dialog = create();
            dialog.show();
            return dialog;
        }


客户端代码用法如下:


new AlertDialog.Builder(context)
				.setTitle("提示")
				.setMessage("网络连接未打开")
				.setPositiveButton("前往打开",
						new DialogInterface.OnClickListener() {

							public void onClick(DialogInterface dialog,
									int which) {
								Intent intent = new Intent(
										android.provider.Settings.ACTION_WIRELESS_SETTINGS);
								startActivity(intent);
							}
						}).setNegativeButton("取消", null).create().show();


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