????????????????????????????????????????????????????????????????????????????????????????о?String????????????????????????
????/**
????* AbstractStringBuilder??StringBuilder??StringBuffer???????
????*AbstractStringBuilder??append()????
????*/
????public AbstractStringBuilder append(String str) {
????if (str == null)
????return appendNull();
????int len = str.length();
????ensureCapacityInternal(count + len);
????str.getChars(0?? len?? value?? count);
????count += len;
????return this;
????}
????/**
????* StringBuilder???append()????
????*/
????@Override
????public StringBuilder append(String str) {
????super.append(str);
????return this;
????}
????????????????????????????????￡??????л?????????????????????????????????????????????????????????????
????JDK???????????????????????д????????????????????????????2????????????????????????????????????????????????????е?????????????????????????????????????飺
????//????????????????????
????public class Foo {
????}
????class Bar extends Foo{
????}
????/**
????* People????????????????????
????*/
????public class TestPeople {
????Foo  test(Foo f){
????return new Foo();
????}
????}
????public class TestStudent extends TestPeople {
????/**
????* ???????Bar???????з?????Foo???????????????????????????????????
????* @param foo
????* @return
????*/
????@Override  //?????????????????????????????????
????Bar test(Foo foo) {
????return new Bar();
????}
????}