this keyword in java
this
keyword in java
There can be a lot of usage of java this keyword. In
java, this is a reference
variable that
refers to the current object.
Usage of java this keyword
Here is given the 6 usage of java this keyword.
- this
can be used to refer current class instance variable.
- this
can be used to invoke current class method (implicitly)
- this()
can be used to invoke current class constructor.
- this
can be passed as an argument in the method call.
- this
can be passed as argument in the constructor call.
- this
can be used to return the current class instance from the method.
Suggestion: If you
are beginner to java, lookup only three usage of this keyword.
1) this:
to refer current class instance variable
The this keyword can be used to refer current class instance variable. If
there is ambiguity between the instance variables and parameters, this keyword
resolves the problem of ambiguity.
Understanding the problem without this
keyword
Let's
understand the problem if we don't use this keyword by the example given
below:
class
Student{
int
rollno;
String
name;
float
fee;
Student(int
rollno,String name,float fee){
rollno=rollno;
name=name;
fee=fee;
}
void
display(){System.out.println(rollno+" "+name+"
"+fee);}
}
class
TestThis1{
public
static void main(String args[]){
Student
s1=new Student(111,"ankit",5000f);
Student
s2=new Student(112,"sumit",6000f);
s1.display();
s2.display();
}}
Output
0 null
0.0
0 null
0.0
In the
above example, parameters (formal arguments) and instance variables are same.
So, we are using this keyword to distinguish local variable and instance
variable.
Solution of the above problem by this
keyword
class
Student{
int
rollno;
String
name;
float
fee;
Student(int
rollno,String name,float fee){
this.rollno=rollno;
this.name=name;
this.fee=fee;
}
void
display(){System.out.println(rollno+" "+name+"
"+fee);}
}
class
TestThis2{
public
static void main(String args[]){
Student
s1=new Student(111,"ankit",5000f);
Student
s2=new Student(112,"sumit",6000f);
s1.display();
s2.display();
}}
1.
Output:
2.
3.
111 ankit 5000
4.
112 sumit 6000
5.
If local variables(formal arguments) and instance variables are
different, there is no need to use this keyword like in the following
program:
6.
7.
Program where this keyword is not required
1.
class Student{
2.
int rollno;
3.
String name;
4.
float fee;
5.
Student(int r,String n,float f){
6.
rollno=r;
7.
name=n;
8.
fee=f;
9.
}
10.
void display(){System.out.println(rollno+" "+name+" "+fee);}
11.
}
12.
8.
class TestThis3{
13.
public static void main(String args[]){
14.
Student s1=new Student(111,"ankit",5000f);
15.
Student s2=new Student(112,"sumit",6000f);
16.
s1.display();
17.
s2.display();
18.
}}
9.
Output:
10.
11.
111 ankit 5000
12.
112 sumit 6000
|
It is better approach to use meaningful
names for variables. So we use same name for instance variables and parameters
in real time, and always use this keyword.
2) this: to invoke current class method
You may invoke the method of the current class by using the this
keyword. If you don't use the this keyword, compiler automatically adds this
keyword while invoking the method. Let's see the example
class A{
void m(){System.out.println("hello m");}
void n(){
System.out.println("hello n");
//m();//same as this.m()
this.m();
}
}
class TestThis4{
public static void main(String args[]){
A a=new A();
a.n();
}}
Output:
hello n
hello m
3) this() : to invoke current class constructor
The this() constructor call can be used to invoke the current
class constructor. It is used to reuse the constructor. In other words, it is
used for constructor chaining.
Calling default constructor from parameterized constructor:
class A{
A(){System.out.println("hello a");}
A(int x){
this();
System.out.println(x);
}
}
class TestThis5{
public static void main(String args[]){
A a=new A(10);
}}
Output:
hello a
10
Calling parameterized constructor from default constructor:
class A{
A(){
this(5);
System.out.println("hello a");
}
A(int x){
System.out.println(x);
}
}
class TestThis6{
public static void main(String args[]){
A a=new A();
}}
Output:
5
hello a
Real
usage of this() constructor call
The this() constructor call should be used to reuse the constructor from the
constructor. It maintains the chain between the constructors i.e. it is used
for constructor chaining. Let's see the example given below that displays the
actual use of this keyword.
class Student{
int rollno;
String name,course;
float fee;
Student(int rollno,String name,String course){
this.rollno=rollno;
this.name=name;
this.course=course;
}
Student(int rollno,String name,String course,float
fee){
this(rollno,name,course);//reusing constructor
this.fee=fee;
}
void display(){System.out.println(rollno+"
"+name+" "+course+" "+fee);}
}
class TestThis7{
public static void main(String args[]){
Student s1=new
Student(111,"ankit","java");
Student s2=new
Student(112,"sumit","java",6000f);
s1.display();
s2.display();
}}
Output:
111 ankit java null
112 sumit java 6000
Rule: Call to this()
must be the first statement in constructor.
class Student{
int rollno;
String
name,course;
float fee;
Student(int
rollno,String name,String course){
this.rollno=rollno;
this.name=name;
this.course=course;
}
Student(int
rollno,String name,String course,float fee){
this.fee=fee;
this(rollno,name,course);//C.T.Error
}
void
display(){System.out.println(rollno+" "+name+"
"+course+" "+fee);}
}
class TestThis8{
public static void
main(String args[]){
Student s1=new
Student(111,"ankit","java");
Student s2=new
Student(112,"sumit","java",6000f);
s1.display();
s2.display();
}}
Compile Time Error:
Call to this must be first statement in constructor
4) this:
to pass as an argument in the method
class S2{
void m(S2 obj){
System.out.println("method is
invoked");
}
void p(){
m(this);
}
public static void main(String args[]){
S2 s1 = new S2();
s1.p();
}
}
Output
method is invoked
Application of this
that can be passed as an argument:
In event handling
(or) in a situation where we have to provide reference of a class to another
one. It is used to reuse one object in many methods.
5) this: to pass as
argument in the constructor call
We can pass the this
keyword in the constructor also. It is useful if we have to use one object in
multiple classes. Let's see the example:
class B{
A4 obj;
B(A4 obj){
this.obj=obj;
}
void display(){
System.out.println(obj.data);//using data
member of A4 class
}
}
class A4{
int data=10;
A4(){
B b=new B(this);
b.display();
}
public static void main(String args[]){
A4 a=new A4();
}
}
Output:10
6) this keyword can be used to return current class
instance
We can return this keyword as an statement from the method. In
such case, return type of the method must be the class type (non-primitive).
Let's see the example:
Syntax
of this that can be returned as a statement
return_type
method_name(){
return this;
}
Example of this keyword that you return as a statement from the method
class A{
A getA(){
return this;
}
void msg(){System.out.println("Hello java");}
}
class Test1{
public static void main(String args[]){
new A().getA().msg();
}
}
Output:
Hello java
Proving
this keyword
Let's
prove that this keyword refers to the current class instance variable. In
this program, we are printing the reference variable and this, output of both
variables are same.
|
class A5{
void m(){
System.out.println(this);//prints same reference ID
}
public static void main(String args[]){
A5 obj=new A5();
System.out.println(obj);//prints the reference ID
obj.m();
}
}
Output:
A5@22b3ea59
A5@22b3ea59
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