每次看到Java中的this指針，總摸不著頭緒。在網上看了很多人的講解，還是不知道this指針到底是什么東西，今天的的這篇日志可以讓你看清this到底是誰。(內容摘自：http://www.mathcs.emory.edu/~cheung/Courses/170.2010/Syllabus/03/implicit-param.html)

抽象的講，this指針就是一個Implicit Parameter，那到底什么是Implicit Parameter，請看下文詳解。

Implicit Parameter

Parameters to an?instance?method

Recall that a?parameter?is a value that is given to a method as input - see:?click here

Methods can have?one or moreparameters

You can pass one or more inputs into a method by specifying the value as parameters when you invoke the method.

We have learned that?instance methods?have?two different kinds?of parameters:

Explicit?parameter?that is passed by specifying the?parameter?in the parenthesis of a method call.

Implicit?parameter?that is passed by specifying an?object variable (object reference)?before the name of a method.

Explicitparameter

Review:

The?parameters between the brackets/parenthesisof the method call are called?explicit?parameters

Examples:?the values in?red?are?explicit parameters

harrysChecking.deposit(500);

harrysChecking.withdraw(amount);

box.translate(10, 20);

double x = 4.0, y = 6.0;

box.translate(x, y);

NOTE: these statement must be contained in some method, and most likely, they will be in different methods (because their actions are kinda "unrelated". I have omitted the method for brevity - just want to show you how to identify?explicit parameters

Accessing an?explicit?parameter variable inside the method

Accessing an?explicit?parameter variable inside a method is very simple:

To use an?explicit?parameter variable inside a method, you simply refer the parameter variable by its name

Example:

public void deposit(double amount)

{

balance = balance + amount;

}

(Because this is so trivial, I did not dwell on it when we discuss the implementation of the methods.

I dwell on it now because I will show you how to access the?implicit?parameter variable next.)

Implicit?parameter

Review:

The?object variable before the method namein the method call are called?implicit?parameters

The?object?on which you invoke a method is also as a?parameter?in the method call, because if you use a different object in the method call, the operation is performed on a different object (i.e., the behavior of the method is modified):

harrysChecking.deposit(500);

momsSaving.deposit(500);

The first call updates the?balance?instance variable in?harrysChecking, while the second call updates the?balance?instance variable in?momsSaving

Accessing the?Implicit?parameter inside the method

There is?exactly ONE?implicit?parameter variable in every method.

Java has assigned a?special name (a keyword)?to identify this?implicit?parameter variable

The?keyword?that Java (and C++) uses to refer to the?implicit?parameter variable is called?this

It must be written with all?lower case letters

The obvious question that you will be asking is:

Question:

What value does the?implicit?parameter variable?this?contain ???

You can figure the answer to this question by using analogy...

Consider the following two pieces of program fragments:

public void deposit(double amount)

{

balance = balance + amount;

}

(1) harrysChecking.deposit(700);

(2) momsSaving.deposit(500);

Question:

What is the value of the?(explicit?parameter variable)?amountin the case (1) ?

amount= 700

Question:

And what is the value of the?(explicit?parameter variable)?amountin the case (2) ?

amount= 500

OK, it's time to apply the analogy:

Question:

What is the value of the?(implicit?parameter variable)?thisin the case (1) ?

this= harrysChecking !!!

Question:

And what is the value of the?(implicit?parameter variable)?thisin the case (2) ?

this= momsSaving !!!

Here is a pictorial representation on what's going on inside the computer when?momsSaving.deposit(500)?is called:

As you know, object variables such as?harrysChecking?and?momsSaving?are used to locate the?instance variables?of an object - see:?click here

Since the object variable (harrysChecking?in case (1) and?momsSaving?in case (2)) is passed to the?implicit?parameter variable?this?in the method call, the method can use?this?to obtain the?correct?instance variables?!!!

(That's how the magic works....)

Important Fact:

The?implicit?parameter variable?this?is an?object reference variable?and is used to locate the instance variables of the object

It remains to show you?how?the?implicit?parameter variable?this?is used...

Example:

public void deposit(double amount)

{

balance = balance + amount;

}

public void deposit(double amount)

{

this.balance = this.balance + amount;

}

Notice that the variable?balance?is an instance variable in the object that is being referenced by the?implicit?parameter variable?this

Java assumes that a variable name that?does not any a name of a?parameter?variable or a?local?variablemust refer to?an instance variable

That's why you don't need to write?this?before the variable?balance

A case that necessitate the use of?this

It is?rare?that you need to use the?implicit?parameter variable?this

Here is one case where it is necessary to use?this, but it is only so, because of a very bad choice of nomenclature:

the name of (one of the) parameter variable?is the sameas the name of the instance variable

Example where you need to use?this?in the method:

public void deposit(double amount)

{

balance = balance + amount;

}

public void deposit(double balance)

{

balance = balance + balance;

}

The?deposit()?on the right will not work properly....

You can fix this by using?this:

public void deposit(double balance)

{

this.balance = this.balance + balance;

}

DEMO Program:(BankAccount class with?this) ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??

BankAccount.java file:?click here

BankAccountTester.java file:?click here

Compile with: ??javac BankAccountTester.java

Run with: ??java BankAccountTester

Now edit?BankAccount.java?and remove the?this., compile and run the program again...

The method?deposit()?can no longer increase the balance in a?BankAccount?object...

Question: WHY NOT ???

The name?balance?matches the name of a?parameter variableand thus, the name?balance?refers to the?parameter variableand?not?to the?instance variable

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