15--Object class related exercises

The first question: the equals method of the Object class

• Briefly describe the difference between the equals method in the String class and the equals method in the Object class.

   String类中的equals方法是用来判断两个对象的内容是否相同，
   而Object 类中的equals方法是用来判断两个对象是否是同一个
   对象，所谓同一个对象指的是内存中的同一块存储空间。
  

The second question: the toString method of the Object class

• Do not run the code, just tell the print result and explain the reason.

public class ToStringTest{
    
    

    static int i = 1;

    public static void main(String args[]){
    
    

        System.out.println("love " + new ToStringTest());//love java

        ToStringTest a = new ToStringTest();

        a.i++;

        System.out.println("me " + a.i);//me 2

    }

    public String toString(){
    
    

        System.out.print("I ");//I

        return "java ";

    }

}

Operation result: I love java me 2

原因：当执行代码的时候，首先加载静态变量，然后执行main方法，
由于main方法内部第一行代码为输出语句，里面new了此类对象，
当执行此行代码时会先创建了本类的对象，由于此类重写了toString
方法，会先执行toString方法的打印输出，然后返回“java ”，再执行main
方法第一行打印输出。在Java中“System.out.println(类对象名);”实际输出
的是该对象的toString()方法返回的字符串，即括号中的内容等价于类对
象名.toString(),toString方法的好处是在碰到println方法的时候会被自动调用
，不用显示的写出来。

The third question: the equals method of the Object class

• Look at the following program, do not run and say the result, write the answer, and run it in IntelliJ IDEA to see if the answer and running result are correct, and analyze the reason.

 (1)
               String s1 = new String("abc");

                 String s2 = "abc";

                 System.out.println(s1 == s2);             //false

                 System.out.println(s1.equals(s2));        //true
         (2)
               String s1 = "abc";

                 String s2 = "abc";

                 System.out.println(s1 == s2);             //true

                 System.out.println(s1.equals(s2));        //true
         (3)
               String s1 = "a" + "b" + "c";

                 String s2 = "abc";

                 System.out.println(s1 == s2);              //true

                 System.out.println(s1.equals(s2));         //true
         (4)
               String s1 = "ab";

                 String s2 = "abc";

                 String s3 = s1 + "c";

                 System.out.println(s3 == s2);             //false

                 System.out.println(s3.equals(s2));        //true