[Java Foundation] Detailed explanation of method technology in Java Object

Methods in Object

The Object class is the parent class of all classes, and all classes inherit Object by default at the bottom

equals returns true, hashCode must be the same, so hashCode and equals must be rewritten at the same time (applied in the set)

equals(Object obj): Determine whether the objects are the same

Rewriting equals() method must rewrite hashCode()

• When overriding equals(), if the object contains object properties

  • Or initialize the object properties when creating the object

  • Or judge whether the object attributes are all null in the equals() method;

        @Override
        public boolean equals(Object o) {
            
            
            if (this  o) return true;
            if (o  null || getClass() != o.getClass()) return false;
            Person person = (Person) o;
            return age  person.age &&
                    gender  person.gender &&
                    (namenull && person.namenull)|| name.equals(person.name);
        }
    

• What is returned is the hash code value of the object

• Different objects have different hash code values

• Hash code value: Hash code value is almost never repeated -----Hash code value must be unique-----represents the address value of the object

  • Value range: as large as the int range, the hash value is not negative [0-41 billion]
  • Hash distribution: evenly distributed

toString(): Return the object as a string

Do not rewrite: the toString() in the Object class is called, and the splicing address value of the object is returned

Rewrite: call the rewritten toString() method

• System.out.println(demo3);And System.out.println(demo3.toString());the same effect
  • The bottom layer defaults to let the object call toString() in the Object class to return the address value of the object

clone(): Create and return a copy of this object

• Need to inherit the Cloneableinterface (this interface is only used as a mark) to call the clone()method

• clone()The method can only be called in the Object subclass itself. Calling methods of other objects in other classes clone()will cause an error. Reason: protected feature

• CloneNotSupportedException: Cloning does not support exceptions (does not implement the cloneable interface)

Clone code example

package cn.tedu.objectdemo;

/**
 * Cloneable接口没有内容，仅仅只是作为一个标记
 * 类实现Cloneable接口，类产生的对象就能支持克隆操作
*/
public class ObjectDemo1 implements Cloneable{
     
     
 int i = 6;
 public static void main(String[] args) throws CloneNotSupportedException {
     
     
     //创建对象
     ObjectDemo1 od = new ObjectDemo1();
     //调用克隆方法
     ObjectDemo1 od1 = (ObjectDemo1) od.clone();
     //克隆对象调用属性
     System.out.println(od1.i);
     System.out.println(odod1);
 }
}

finalize(): Notify the system for recycling

When the garbage collection determines that there is no reference to the object, it is called by the garbage collector on the object.

getClass(): Returns the class of Object runtime

Package name + class name: full path name

Which object is called returns the full path name of the object

Example code

Student class

package demoObject;
 
public class Student {
    
    
	private int age;
 
	public int getAge() {
    
    
		return age;
	}
 
	public void setAge(int age) {
    
    
		this.age = age;
	}
 
	public Student(int age) {
    
    
		super();
		this.age = age;
	}
	public Student() {
    
    
 
	}
	
 
	@Override
	public String toString() {
    
    
		return "Student [age=" + age + "]";
	}
 //重写hashCode和equals方法------------------------------------
	@Override
	public int hashCode() {
    
    
		final int prime = 31;
		int result = 1;
		result = prime * result + age;
		return result;
	}
 
	@Override
	public boolean equals(Object obj) {
    
    
		if (this  obj)
			return true;
		if (obj  null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Student other = (Student) obj;
		if (age != other.age)
			return false;
		return true;
	}
}

DemoObject test class

package demoObject;
 
public class DemoObject {
    
    
	//toString()方法
	public static void test1() {
    
    
		Student stu = new Student(20);
		String str = new String("Hello");
		System.out.println(stu);
		System.out.println(str);
	}
	//hashCode()方法
	public static void test2() {
    
    
		Student stu1 = new Student(20);
		Student stu2 = new Student(20);
		String str1 = new String("Hello");
		String str2 = new String("Hello");
		System.out.println(stu1.hashCode()+","+stu2.hashCode());
		System.out.println(str1.hashCode()+","+str2.hashCode());
	}
	//equals()方法
	public static void test3() {
    
    
		Student stu1 = new Student(20);
		Student stu2 = new Student(20);
		String str1 = new String("Hello");
		String str2 = new String("Hello");
		System.out.println(stu1.equals(stu2));
		System.out.println(str1.equals(str2));
	}
 
	public static void main(String[] args) {
    
    
		test3();
	}
 
}