What is the usage of this keyword in java

Usage of this keyword in java: 1. Call the attributes in this class, that is, the member variables in the class; 2. Call other methods in this class; 3. Call other constructors in this class , it needs to be placed in the first line of the constructor when called.

This keyword has three main applications:

this calls the attributes in this class, which is the member variables in the class;

This calls other methods in this class;

This calls other constructors in this class. The call should be placed in the first line of the constructor.

Calling member variables

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It will be more specific when combined with the code. Refer to the following code:

public class Student{
    String name;
    private void SetName(String name){
        this.name = name;
    }
}

In this code, a class Student is created, with a member variable name and a member method SetName (String name). Since the formal parameter name received by the member method is the same as the member The variables are the same, both are names, so you can use the this keyword here to call member variables in this class. Its function can be simply understood as follows: the this keyword is to call the member variables in this class.

But such a simple understanding is not accurate. This is actually a reference to the current class object. The member variables of the object are accessed through the reference to the object instantiated by this class.

Note:

This keyword is not subject to access permission control when accessing member variables and member functions of a class. It can access all member variables and methods in this class, including private member variables. and methods. You can also access static members of this class through this. However, since static members can be directly accessed through the class name, if you access through this, there will be a warning message "The static field ××× should be accessed in a static way". This cannot be used within a static member of a class or within a static block.

Calling member methods

You can refer to the following code:

package Test;

public class ThisTest {
    String name;

    private void setName(String name) {
        this.name = name;
        System.out.println("setName: " + this.name);
    }

    private void thisTest(String name) {
        this.setName(name);
        System.out.println("thisTest: " + this.name);
    }

    public static void main(String[] args) {
        ThisTest tt = new ThisTest();

        tt.setName("Yida");
        tt.thisTest("Jenny");

    }
}

In this code, the Test class is first created. This class is no different from the previous one, except that a Member method thisTest(), in this member method, the previous member function setName() is called through the this keyword. The program running result is as follows:

setName: Yida
setName: Jenny
thisTest: Jenny

Call the constructor

Construction A method is automatically called when an object of a class is created through the new keyword. It cannot be called through the method name (that is, the class name) like other methods in the program. But if a class has multiple constructors, you can call other constructors through this (paras...) in one constructor.

Using this to call other constructors has the following constraints:

You can only call other constructors through this in the constructor, and cannot use it in ordinary methods.

The constructor cannot be called recursively through this, that is, the constructor itself cannot be called directly or indirectly through this in a constructor.

Calling other constructors through this must be executed in the first line of the constructor.

Since super calls the constructor of the parent class must also be executed in the first line of the constructor, therefore, calling the constructor through this and super cannot appear in the same constructor at the same time. You cannot call different constructors multiple times in one constructor.

The following code is wrong because a recursive call is formed through the this keyword:

//错误的示例！！！
class test {
    test() {
        this(1);
    }
    test(int a){
        this();
    }
    tes int b) {
        this(1, 2);
    }
}

The correct usage is as follows:

public class Student { //定义一个类，类的名字为student。 
    public Student() { //定义一个方法，名字与类相同故为构造方法
        this(“Hello!”);
    }
    public Student(String name) { //定义一个带形式参数的构造方法
    }
}

Return the value of the object

In addition to referencing variables or member methods, this keyword also has an important role in returning a reference to the class. For example, in code, you can use return this to return a reference to a certain class. At this time, the this keyword represents the name of the class. If the code is in the student class above, then the code represents return student. It can be seen that in addition to referencing variables or member methods, the this keyword can also be used as the return value of the class. This is the most eye-catching part of the this keyword.

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