Java study notes: to_String (), equals ()

Java learning to_String (), equals ()

• to_String:

  • Returns a string

  • Any class inherits the Object class by default, and the toString () method is inside the Object

  • ToString () can be rewritten

class A
{
}
class TestA
{
    public static void main(String[] args)
    {
        A aa = new A();
        System.out.printf("%s \n", aa.toString());
        //打印出的结果是 A@de3ca2f,A是类名，后面6/8个是类对象地址的16进制  P58讲
        //若改为("%s \n", aa); 输出结果一样
    }
}

• equals:

  • equals () is also in class Object

  • So all classes can call equals

  • It is used to determine whether the memory addresses of the two objects are the same, and the return value is true / false

class A
{
    public A(int i)
    {
        this.i = i;
    }
}
class Testequals
{
    public static void main(String[] args)
    {
        A a1 = new A(2);
        A a2 = new A(2);
        System.out.println( a1.equals(a2));//判断a1 a2内存地址是否相同
        //虽然两个对象所指堆中保存的值一样，但是地址不同
        // 会返回 false
    }
}

• How to rewrite equals ():
  The purpose of rewriting equals is to make two comparisons the same, that is, return true

Rewritten as follows:

class A
{
    public int i;
    public A(int i)
    {
        this.i = i;
    }
    public boolean equals(Object obj)  //一定要有public
    {
        //因为上面传入的参数a2本身已是子类对象，
        //所以这里不用写 Obj aa == new A(); 
        A aa = (A)obj;  //将父类对象obj(此时obj是a2子类对象/引用)强制转换成子类的引用
        if(this.i == aa.i)  //if ( 当前对象的 i 与 obj 的i相等)
            return true;
        else
            return false;
    }
}

• Note:
  if (this.i == obj.i) cannot be written above, because the parent class object cannot directly call the specific properties / methods (members) of the child class

Test to see if you have rewritten equals:

String a1 = "123";
String a2 = "123";
System.out.println(a1.equals(a2));
//返回的是true， 说明类String 中的equals方法已经重写了 

For strings, equals () compares what the string object points to, not the object itself.