public String substring(int beginIndex)
public String substring(int beginIndex, int endIndex)

public String substring(int beginIndex, int endIndex) {
if (beginIndex < 0) {
throw new StringIndexOutOfBoundsException(beginIndex);
}
if (endIndex > count) {
throw new StringIndexOutOfBoundsException(endIndex);
}
if (beginIndex > endIndex) {
throw new StringIndexOutOfBoundsException(endIndex - beginIndex);
}
return ((beginIndex == 0) && (endIndex == count)) ? this :
new String(offset + beginIndex, endIndex - beginIndex, value);
}

在方法的最后返回了一个新建的String对象。该String的构造函数如 下：

// 包内可见的构造器，通过共享数组内容加速访问
String(int offset, int count, char value[]) {
this.value = value;
this.offset = offset;
this.count = count;
}