我什么时候应该在 class 中使用“this”？

Question

I know that this refers to a current object. But I do not know when I really need to use it. For example, will be there any difference if I use x instead of this.x in some of the methods? May be x will refer to a variable which is local for the considered method? I mean variable which is seen only in this method.

What about this.method() ? Can I use it? Should I use it. If I just use method() , will it not be, by default, applied to the current object?

Answer 1

The this keyword is primarily used in three situations. The first and most common is in setter methods to disambiguate variable references. The second is when there is a need to pass the current class instance as an argument to a method of another object. The third is as a way to call alternate constructors from within a constructor.

Case 1: Using this to disambiguate variable references. In Java setter methods, we commonly pass in an argument with the same name as the private member variable we are attempting to set. We then assign the argument x to this.x . This makes it clear that you are assigning the value of the parameter "name" to the instance variable "name".

public class Foo
{
    private String name;

    public void setName(String name) {
        this.name = name;
    }
}

Case 2: Using this as an argument passed to another object.

public class Foo
{
    public String useBarMethod() {
        Bar theBar = new Bar();
        return theBar.barMethod(this);
    }

    public String getName() {
        return "Foo";
    }
}

public class Bar
{
    public void barMethod(Foo obj) {
        obj.getName();
    }
}

Case 3: Using this to call alternate constructors. In the comments, trinithis correctly pointed out another common use of this . When you have multiple constructors for a single class, you can use this(arg0, arg1, ...) to call another constructor of your choosing, provided you do so in the first line of your constructor.

class Foo
{
    public Foo() {
        this("Some default value for bar");

        //optional other lines
    }

    public Foo(String bar) {
        // Do something with bar
    }
}

I have also seen this used to emphasize the fact that an instance variable is being referenced (sans the need for disambiguation), but that is a rare case in my opinion.

Answer 2

The second important use of this (beside hiding with a local variable as many answers already say) is when accessing an outer instance from a nested non-static class:

public class Outer {
  protected int a;

  public class Inner {
    protected int a;

    public int foo(){
      return Outer.this.a;
    }

    public Outer getOuter(){
      return Outer.this;
    }
  }
}

Answer 3

You only need to use this<\/code> - and most people only use it - when there's an overlapping local variable with the same name. (Setter methods, for example.)

Answer 4

The only need to use the this. qualifier is when another variable within the current scope shares the same name and you want to refer to the instance member (like William describes). Apart from that, there's no difference in behavior between x and this.x .

Answer 5

"this" is also useful when calling one constructor from another:

public class MyClass {
    public MyClass(String foo) {
        this(foo, null);
    }
    public MyClass(String foo, String bar) {
        ...
    }
}

Answer 6

There are a lot of good answers, but there is another very minor reason to put this<\/code> everywhere. If you have tried opening your source codes from a normal text editor (eg notepad etc), using this<\/code> will make it a whole lot clearer to read.

public class Hello {
    private String foo;

    // Some 10k lines of codes

    private String getStringFromSomewhere() {
        // ....
    }

    // More codes

    public class World {
        private String bar;

        // Another 10k lines of codes

        public void doSomething() {
            // More codes
            foo = "FOO";
            // More codes
            String s = getStringFromSomewhere();
            // More codes
            bar = s;
        }
    }
}

Answer 7

this<\/code> is useful in the builder pattern.

public class User {

    private String firstName;
    private String surname;

    public User(Builder builder){
        firstName = builder.firstName;
        surname = builder.surname;
    }

    public String getFirstName(){
        return firstName;
    }

    public String getSurname(){
        return surname;
    }

    public static class Builder {
        private String firstName;
        private String surname;

        public Builder setFirstName(String firstName) {
            this.firstName = firstName;
            return this;
        }

        public Builder setSurname(String surname) {
            this.surname = surname;
            return this;
        }

        public User build(){
            return new User(this);
        }

    }

    public static void main(String[] args) {
        User.Builder builder = new User.Builder();
        User user = builder.setFirstName("John").setSurname("Doe").build();
    }

}

Answer 8

除非您有重叠的变量名称，否则在阅读代码时它实际上只是为了清楚起见。

"

Answer 9

@William Brendel answer provided three different use cases in nice way.

Answer 10

Google turned up a page on the Sun site that discusses this a bit.

You're right about the variable; this can indeed be used to differentiate a method variable from a class field.

    private int x;
    public void setX(int x) {
        this.x=x;
    }

However, I really hate that convention. Giving two different variables literally identical names is a recipe for bugs. I much prefer something along the lines of:

    private int x;
    public void setX(int newX) {
        x=newX;
    }

Same results, but with no chance of a bug where you accidentally refer to x when you really meant to be referring to x instead.

As to using it with a method, you're right about the effects; you'll get the same results with or without it. Can you use it? Sure. Should you use it? Up to you, but given that I personally think it's pointless verbosity that doesn't add any clarity (unless the code is crammed full of static import statements), I'm not inclined to use it myself.

Answer 11

Following are the ways to use 'this' keyword in java :

1. Using this keyword to refer current class instance variables
2. Using this() to invoke current class constructor
3. Using this keyword to return the current class instance
4. Using this keyword as method parameter

https://docs.oracle.com/javase/tutorial/java/javaOO/thiskey.html

Answer 12

when there are two variables one instance variable and other local variable of the same name then we use this. to refer current executing object to avoid the conflict between the names.

Answer 13

this is a reference to the current object. It is used in the constructor to distinguish between the local and the current class variable which have the same name. eg:

public class circle {
    int x;
    circle(int x){
        this.x =x;
        //class variable =local variable 
    }
} 

this can also be use to call one constructor from another constructor. eg:

public class circle {
    int x;

    circle() { 
        this(1);
    }

    circle(int x) {
        this.x = x; 
    }
}

Answer 14

Will be there any difference if I use "x" instead of "this.x" in some of the methods?

  class A {
     private int i;
     public A(int i) {
        this.i = i; // this.i can be used to disambiguate the i being referred to
     }
  }

Answer 15

this<\/code> does not affect resulting code - it is compilation time operator and the code generated with or without it will be the same. When you have to use it, depends on context. For example you have to use it, as you said, when you have local variable that shadows class variable and you want refer to class variable and not local one.

class POJO {
   protected int i;

   public void modify() {
      i = 9;
   }

   public void thisModify() {
      this.i = 9;
   }
}

Answer 16

With respect to William Brendel<\/a> 's posts and dbconfessions<\/a> question, regarding case 2<\/strong> . Here is an example:

public class Window {

  private Window parent;

  public Window (Window parent) {
    this.parent = parent;
  }

  public void addSubWindow() {
    Window child = new Window(this);
    list.add(child);
  }

  public void printInfo() {
    if (parent == null) {
      System.out.println("root");
    } else {
      System.out.println("child");
    }
  }

}

Answer 17

“This” keyword in java is used to refer current class objects.<\/strong>

Answer 18

To make sure that the current object's members are used. Cases where thread safety is a concern, some applications may change the wrong objects member values, for that reason this should be applied to the member so that the correct object member value is used.

If your object is not concerned with thread safety then there is no reason to specify which object member's value is used.