java异常中if (!isPositionIndex(index)) throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

public boolean addAll(int index, Collection<? extends E> c) {
        checkPositionIndex(index);
    }
        
 private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    private void checkElementIndex(int index) {
        if (!isElementIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

上述源码出自LinkedList.class.
对于if语句后直接抛出异常看上去还是一脸懵的.

这样写就是可以直接抛出异常信息,根据自己定义的情况来抛出从而终止后面程序的运行.

测试代码:

List list = new LinkedList();
    list.add(2);
    list.add(12);
    System.out.println(list.size());
    if (list.size() < 0) {
      throw new IndexOutOfBoundsException("size()<0");
    } else {
      throw new IndexOutOfBoundsException("size()>0");
    }

结果:
​​​1​​​​2​​​​Exception in thread "main" java.lang.IndexOutOfBoundsException: size()>0 at cn.com.clearlight.test.CollectionsDemo.main(CollectionsDemo.java:28)​​ 如果再往下面if-else语句后面写代码的话将出现错误信息:Unreachable code

这样就容易理解源码中的这种写法了.