java中泛型创建数组的总结

在java中,不能通过直接通过T[] tarr=new T[10]的方式来创建数组,最简单的方式便是通过Array.newInstance(Class<t>type,int size)的方式来创建数组例如下面的程序

public class ArrayMaker<T> {
    private Class<T> type;

    public ArrayMaker(Class<T> type) {
        this.type = type;
    }

    @SuppressWarnings("unchecked")
    T[] createArray(int size) {
        return (T[]) Array.newInstance(type, size);
    }

    List<T> createList() {
        return new ArrayList<T>();
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        /*
         * Even though kind is stored as Class<T> , erasure means that it is actually just being stored as a Class, with
         * no parameter. So, when you do some thing with it, as in creating an array, Array.newInstance( ) doesn’t
         * actually have the type information that’s implied in kind; so it cannot produce the specific result, wh ich
         * must therefore be cast, which produces a warning that you cannot satisfy.
         */
        ArrayMaker<Type> am2 = new ArrayMaker<Type>(Type.class);
        System.out.println(Arrays.asList(am2.createArray(10)));
        System.out.println(Arrays.asList(am2.createList()));
    }

}

class Type {
    @Override
    public String toString() {
        return "type";
    }
}

上面的这个例子比较简单,但是如果你有接触过泛型数组,你便对他的复杂度有一定的了解,由于创建泛型数组比较复杂,所以在实际的应用过程中一般会选择List的对泛型进行存储,如果实在需要使用泛型数组,则需要注意数组的在运行时的类型,think in java这本书中,对泛型数组的处理通过四个小程序对其进行了比较完整的描述

程序一:这个程序主要说明了,在使用泛型数组中容易出现的问题,由于书中对于程序的说明比较详细,所以只对程序做引用

class Generic<T> {
}

public class ArrayofGeneric {
    public static void main(String[] args) {
        Generic<Integer>[] genArr;
        /*
         * will throw ClassCastException :The problem is that arrays keep track of their actual type, and that type is
         * established at the point of creation of the array. So even though genArr has been cast to a Generic < Integer
         * >[] , that information only exists at compile time (and without the @SuppressWarnings annotation, you’d get a
         * warning for that cast). At run time, it’s still an array of Object, and that causes problems.
         */
        // genArr = (Generic<Integer>[]) new Object[] {};
        /* can not create a generic of array */
        // genArr=new Generic<Integer>[2];
        genArr = (Generic<Integer>[]) new Generic[2];
        System.out.println(genArr);
    }
}

程序二:这个程序主要是说明在程序的执行过程中,泛型数组的类型信息会被擦除,且在运行的过程中数组的类型有且仅有Object[],如果我们强制转换成T[]类型的话,虽然在编译的时候不会有异常产生,但是运行时会有ClassCastException抛出

/**
 *
 * Because of erasure, the runtime type of the array can only be Object[]. If we immediately cast it to T[], then at
 * compile time the actual type of the array is lost, and the compiler may miss out on some potential error checks.
 *
 *
 *
 * archive $ProjectName: $
 *
 * @author Admin
 *
 * @version $Revision: $ $Name: $
 */
public class ArrayOfGeneric2<T> {
    public T[] ts;

    public ArrayOfGeneric2(int size) {
        ts = (T[]) new Object[size];
    }

    public T get(int index) {
        return ts[index];
    }

    public T[] rep() {
        return ts;
    }

    public void set(int index, T t) {
        ts[index] = t;
    }

    public static void main(String[] args) {
        ArrayOfGeneric2<String> aog2 = new ArrayOfGeneric2<String>(10);
        Object[] objs = aog2.rep();
        System.out.println(objs);
        /* will throw ClassCastException */
       // String[] strs = aog2.rep();
        // System.out.println(strs);
    }

}

程序三:主要说明在对象中通过用Object[]来保存数据,则生成对象是,可以对其持有的对象在T和object之间进行转换,但是当设计到数组的转换时,还是会报ClassCastException

/**
 *
 * Initially, this doesn’t look very different compare with ArrayOfGeneric2.java , just that the cast has been moved.
 * Without the ©SuppressWarnings annotations, you will still get "unchecked" warnings. However, the internal
 * representation is now Object[] rather than T[]. When get( ) is called, it casts the object to T, which is in fact the
 * correct type, so that is safe. However, if you call rep( ) , it again attempts to cast the Object[] to a T[], which
 * is still incorrect, and produces a warning at compile time and an exception at run time. Thus there’s no way to
 * subvert the type of the underlying array, which can only be Object[]. The advantage of treating array internally as
 * Object[] instead of T[] is that it’s less likely that you’ll forget the runtime type of the array and accidentally
 * introduce a bug (although the majority, and perhaps all, of such bugs would be rapidly detected at run time)
 *
 *
 *
 * archive $ProjectName: $
 *
 * @author Admin
 *
 * @version $Revision: $ $Name: $
 */
public class ArrayOfGeneric3<T> {
    Object[] ts;

    public ArrayOfGeneric3(int size) {
        ts = new Object[size];
    }

    public T get(int index) {
        return (T) ts[index];
    }

    public T[] rep() {
        return (T[]) ts;
    }

    public void set(int index, T t) {
        ts[index] = t;
    }

    public static void main(String[] args) {
        ArrayOfGeneric3<Integer> aog2 = new ArrayOfGeneric3<Integer>(10);
        Object[] objs = aog2.rep();
        for (int i = 0; i < 10; i++) {
            aog2.set(i, i);
            System.out.println(aog2.get(i));
        }
            Integer[] strs = aog2.rep();
            System.out.println(strs);
    }
}

程序四:是对泛型数组相对而言比较完美的解决方案

/**
 *
 * The type token Class<T> is passed into the constructor in order to recover from the erasure, so that we can create
 * the actual type of array that we need, although the warning from the cast must be suppressed with @SuppressWarnings.
 * Once we do get the actual type, we can return it and get the desired results, as you see in main( ). The runtime type
 * of the array is the exact type T[].
 *
 * @author Admin
 *
 * @version $Revision: $ $Name: $
 */
public class ArrayOfGeneric4<T> {

    T[] ts;

    public ArrayOfGeneric4(Class<T> type, int size) {
        /* to solution array of generic key code! */
        ts = (T[]) Array.newInstance(type, size);
    }

    public T get(int index) {
        return ts[index];
    }

    public T[] rep() {
        return ts;
    }

    public void set(int index, T t) {
        ts[index] = t;
    }

    public static void main(String[] args) {
        ArrayOfGeneric4<Integer> aog2 = new ArrayOfGeneric4<Integer>(Integer.class, 10);
        Object[] objs = aog2.rep();
        for (int i = 0; i < 10; i++) {
            aog2.set(i, i);
            System.out.println(aog2.get(i));
        }
        try {
            Integer[] strs = aog2.rep();
            System.out.println("user Array.newInstance to create generci of array was successful!!!!! ");
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

泛型这一章节的内容从擦除开始,觉得都是非常的难懂,如果哪位同志有比较好的建议,希望能不惜指教!