我重写的StringBuiler类：iStringBuilder

public class iStringBuilder {
    String target = new String();
    
    public iStringBuilder(String newString) {
        target = newString;
    }
    
    public void append(iStringBuilder newString) {
        target = target + newString;
    }
    
    public void append(int i) {
        String istr = i + "";
        target = target + istr;
    }
    
    public int length() {
        return (target.length());
    }
    
    public char charAt(int i) {
        return (target.charAt(i));
    }
    
    public String toLowerCase() {
        return (target.toLowerCase());
    }
    
    public String toUpperCase() {
        return (target.toUpperCase());
    }
}

然后是我用来测试类的测试类 Test：

public class Test {

    public static void main(String[] args) {
        iStringBuilder test = new iStringBuilder("HelloWorld");
        iStringBuilder tail = new iStringBuilder("!~~~~~~~");
        System.out.println(test);
        System.out.println(tail);
        
        test.append(tail);
        System.out.println(test);
        test.append(1);
        System.out.println(test);
        System.out.println(test.charAt(5));
        System.out.println(test.toLowerCase());
        System.out.println(test.toUpperCase());
        System.out.println(test.length());
        System.out.println(test.toString());
    }
}

然后是输出：

iStringBuilder@15db9742
iStringBuilder@6d06d69c
iStringBuilder@15db9742
iStringBuilder@15db9742
W
helloworldistringbuilder@6d06d69c1
HELLOWORLDISTRINGBUILDER@6D06D69C1
34
iStringBuilder@15db9742

问题是：打印最初的字符串test时为什么不是HelloWorld而是一个地址，如果说我的方法彻底写错的话那为什么在toLowerCase函数时能打印出helloworld……求诸位指教（我知道String类不可变，但是我写入编译器的时候编译器没有报错，然后翻书百度未果，故来提问……）