数组的subList - 转换为Generic Type时的ClassCastException

Question

我写了一个小实用工具方法,但它总是产生一个ClassCastException,任何想法为什么？以及如何解决它？

<T> T[] subArray(int begin, int end, T[] array) {
    int size = end - begin;
    Object[] newArray = new Object[size];
    for (int i = 0; i < size; i++) {
        newArray[i] = array[begin + i];
    }
    return (T[]) newArray;
}

这是堆栈跟踪:

java.lang.ClassCastException: [Ljava.lang.Object;
at org.robert.distance.framework.FacadeTest.testSubArray(FacadeTest.java:48)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:592)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:76)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:46)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

Answer 1

使用 Arrays#copyOfRange()

<T> T[] subArray(int begin, int end, T[] array) {
    return Arrays.copyOfRange(array, begin, end);
}

哎呀,它甚至使整个subArray()方法变得多余:)它只需要JDK 1.6.如果由于某种原因你还没有,那么这里有一个相关的摘录:

public static <T> T[] copyOfRange(T[] original, int from, int to) {
    int newLength = to - from;
    if (newLength < 0) throw new IllegalArgumentException(from + " > " + to);
    T[] copy = (T[]) Array.newInstance(original.getClass().getComponentType(), newLength);
    System.arraycopy(original, from, copy, 0, Math.min(original.length - from, newLength));
    return copy;
}

Answer 2

我怀疑问题是你要回来了Object[].new Object[size]您需要创建一个正确类型的新数组,而不是使用它.一种方法是使用Array.newInstance(Class <？> componentType,int length).

就像是:

<T> T[] subArray(int begin, int end, T[] array) {
    int size = end - begin;
    T[] newArray = (T[])Array.newInstance(array.getClass().getComponentType(), size);
    for (int i = 0; i < size; i++) {
        newArray[i] = array[begin + i];
    }
    return  newArray;
}