java collection.sort() 根据对象的某个属性对list进行排序

1基本类型的list排序

/** 
 * 对List对象按照某个成员变量进行排序 
 * @param list       List对象 
 * @param sortField  排序的属性名称 
 * @param sortMode   排序方式:ASC,DESC 任选其一 
 */  
public static  void sortList(List list, final String sortField, final String sortMode) {  
    if(list == null || list.size() < 2) {  
        return;  
    }  
    Collections.sort(list, new Comparator() {  
        @Override  
        public int compare(T o1, T o2) {  
            try {  
                Class clazz = o1.getClass();  
                Field field = clazz.getDeclaredField(sortField); //获取成员变量  
                field.setAccessible(true); //设置成可访问状态  
                String typeName = field.getType().getName().toLowerCase(); //转换成小写  
  
                Object v1 = field.get(o1); //获取field的值  
                Object v2 = field.get(o2); //获取field的值  
  
                boolean ASC_order = (sortMode == null || "ASC".equalsIgnoreCase(sortMode));  
  
                //判断字段数据类型,并比较大小  
                if(typeName.endsWith("string")) {  
                    String value1 = v1.toString();  
                    String value2 = v2.toString();  
                    return ASC_order ? value1.compareTo(value2) : value2.compareTo(value1);  
                }  
                else if(typeName.endsWith("short")) {  
                    Short value1 = Short.parseShort(v1.toString());  
                    Short value2 = Short.parseShort(v2.toString());  
                    return ASC_order ? value1.compareTo(value2) : value2.compareTo(value1);  
                }  
                else if(typeName.endsWith("byte")) {  
                    Byte value1 = Byte.parseByte(v1.toString());  
                    Byte value2 = Byte.parseByte(v2.toString());  
                    return ASC_order ? value1.compareTo(value2) : value2.compareTo(value1);  
                }  
                else if(typeName.endsWith("char")) {  
                    Integer value1 = (int)(v1.toString().charAt(0));  
                    Integer value2 = (int)(v2.toString().charAt(0));  
                    return ASC_order ? value1.compareTo(value2) : value2.compareTo(value1);  
                }  
                else if(typeName.endsWith("int") || typeName.endsWith("integer")) {  
                    Integer value1 = Integer.parseInt(v1.toString());  
                    Integer value2 = Integer.parseInt(v2.toString());  
                    return ASC_order ? value1.compareTo(value2) : value2.compareTo(value1);  
                }  
                else if(typeName.endsWith("long")) {  
                    Long value1 = Long.parseLong(v1.toString());  
                    Long value2 = Long.parseLong(v2.toString());  
                    return ASC_order ? value1.compareTo(value2) : value2.compareTo(value1);  
                }  
                else if(typeName.endsWith("float")) {  
                    Float value1 = Float.parseFloat(v1.toString());  
                    Float value2 = Float.parseFloat(v2.toString());  
                    return ASC_order ? value1.compareTo(value2) : value2.compareTo(value1);  
                }  
                else if(typeName.endsWith("double")) {  
                    Double value1 = Double.parseDouble(v1.toString());  
                    Double value2 = Double.parseDouble(v2.toString());  
                    return ASC_order ? value1.compareTo(value2) : value2.compareTo(value1);  
                }  
                else if(typeName.endsWith("boolean")) {  
                    Boolean value1 = Boolean.parseBoolean(v1.toString());  
                    Boolean value2 = Boolean.parseBoolean(v2.toString());  
                    return ASC_order ? value1.compareTo(value2) : value2.compareTo(value1);  
                }  
                else if(typeName.endsWith("date")) {  
                    Date value1 = (Date)(v1);  
                    Date value2 = (Date)(v2);  
                    return ASC_order ? value1.compareTo(value2) : value2.compareTo(value1);  
                }  
                else if(typeName.endsWith("timestamp")) {  
                    Timestamp value1 = (Timestamp)(v1);  
                    Timestamp value2 = (Timestamp)(v2);  
                    return ASC_order ? value1.compareTo(value2) : value2.compareTo(value1);  
                }  
                else {  
                    //调用对象的compareTo()方法比较大小  
                    Method method = field.getType().getDeclaredMethod("compareTo", new Class[]{field.getType()});  
                    method.setAccessible(true); //设置可访问权限  
                    int result  = (Integer)method.invoke(v1, new Object[]{v2});  
                    return ASC_order ? result : result*(-1);  
                }  
            }  
            catch (Exception e) {  
                String err = e.getLocalizedMessage();  
                System.out.println(err);  
                e.printStackTrace();  
            }  
  
            return 0; //未知类型,无法比较大小  
        }  
    });  
}  

2 对自定义对象进行排序

定义对象的代码如下

public class MailSchedule implements Comparable {
    private String mTitile;
    private Long mLongTime;
    private String mTime;
    private String mAddress;
    private boolean mIsFinish;

    public MailSchedule(Long time, String title, String address, boolean isfinish) {
        this.mLongTime = time;
        this.mTitile = title;
        this.mAddress = address;
        this.mIsFinish = isfinish;
    }

    private void getTime(Long time) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy/MM/dd HH:mm");
        String day = sdf.format(new Date(time));

        String date = day.split(" ")[0];
        String mScheduletime = day.split(" ")[1];
        this.mTime = mScheduletime;
    }

    public Long getLongTime() {
        return mLongTime;
    }

    public String getMailScheduleContent() {
        getTime(mLongTime);
        return mTime + " " + mTitile + " " + mAddress;
    }

    public void setIsFinish(boolean isFinish) {
        this.mIsFinish = isFinish;
    }

    public boolean getIsFinish() {
        return mIsFinish;
    }

    public boolean equals(Object obj) {
        if (this == obj) {//如果是引用同一个实例
            return true;
        }
        if (obj != null && obj instanceof MailSchedule) {
            MailSchedule schedule = (MailSchedule) obj;
            return this.mLongTime == schedule.mLongTime && this.mTitile.equals(schedule.mTitile) && this.mAddress.equals(schedule.mAddress);
        } else {
            return false;
        }
    }

    @Override
    public int compareTo(Object comparestu) {
        long compareLongTime = ((MailSchedule) comparestu).mLongTime;
        return this.mLongTime.compareTo(compareLongTime);
    }
}

方法一:对象实现 comparable接口,并实现其compareto方法,通过Collections.sort(currentDayContentList)对装有该对象的list排序

                for(int i= 0;i

方法二:定义一个比较器comparator,通过Collections.sort(currentDayContentList,new comparator())对其list 进行比较

        Collections.sort(currentDayContentList, new Comparator() {
            @Override
            public int compare(MailSchedule lhs, MailSchedule rhs) {
                
                return lhs.getLongTime().compareTo(rhs.getLongTime());
            }
        });

3 Arraylist 中contains方法 如下是源码

    /**
     * Searches this {@code ArrayList} for the specified object.
     *
     * @param object
     *            the object to search for.
     * @return {@code true} if {@code object} is an element of this
     *         {@code ArrayList}, {@code false} otherwise
     */
    @Override public boolean contains(Object object) {
        Object[] a = array;
        int s = size;
        if (object != null) {
            for (int i = 0; i < s; i++) {
                if (object.equals(a[i])) {
                    return true;
                }
            }
        } else {
            for (int i = 0; i < s; i++) {
                if (a[i] == null) {
                    return true;
                }
            }
        }
        return false;
    }

从源码中可以发现,会调用调用所比较对象的equals方法,来判断是否为一个对象的实例!现在要对MailSchedule对象进行比较,可以重写equals方法!

如果我们不在MailSchedule类中重写equals()方法的话,虽然我们用contains方法进行了判断,得到的链表的size仍旧是所new对象的个数,因为我们用new关键字创建MailSchedule对象,虽然它们的内容相同,但是JVM虚拟机在堆中开辟了不同存储空间,分别指向不同对象的引用。

需要我们根据MailSchedule的某个属性进行判断,是否为同一个人,我们需要重写equals方法!代码如上面MailSchedule类中的equals方法


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