Set and Get Methods in java?

How can I use the set and get methods, and why should I use them? Are they rea开发者_如何转开发lly helpful? And also can you give me examples of set and get methods?

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Set and Get methods are a pattern of data encapsulation. Instead of accessing class member variables directly, you define get methods to access these variables, and set methods to modify them. By encapsulating them in this manner, you have control over the public interface, should you need to change the inner workings of the class in the future.

For example, for a member variable:

Integer x;

You might have methods:

Integer getX(){ return x; }
void setX(Integer x){ this.x = x; }

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chiccodoro also mentioned an important point. If you only want to allow read access to the field for any foreign classes, you can do that by only providing a public get method and keeping the set private or not providing a set at all.

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I want to add to other answers that setters can be used to prevent putting the object in an invalid state.

For instance let's suppose that I've to set a TaxId, modelled as a String. The first version of the setter can be as follows:

private String taxId;

public void setTaxId(String taxId) {
    this.taxId = taxId;
}

However we'd better prevent the use to set the object with an invalid taxId, so we can introduce a check:

private String taxId;

public void setTaxId(String taxId) throws IllegalArgumentException {
    if (isTaxIdValid(taxId)) {
        throw new IllegalArgumentException("Tax Id '" + taxId + "' is invalid");
    }
    this.taxId = taxId;
}

The next step, to improve the modularity of the program, is to make the TaxId itself as an Object, able to check itself.

private final TaxId taxId = new TaxId()

public void setTaxId(String taxIdString) throws IllegalArgumentException {
    taxId.set(taxIdString); //will throw exception if not valid
}

Similarly for the getter, what if we don't have a value yet? Maybe we want to have a different path, we could say:

public String getTaxId() throws IllegalStateException {
    return taxId.get(); //will throw exception if not set
}

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I think you want something like this:

public class Person {

  private int age;

  //public method to get the age variable
  public int getAge(){
       return this.age
  }

  //public method to set the age variable
  public void setAge(int age){
       this.age = age;
  }
}

You're simply calling such a method on an object instance. Such methods are useful especially if setting something is supposed to have side effects. E.g. if you want to react to certain events like:

  public void setAge(int age){

       this.age = age;

       double averageCigarettesPerYear = this.smokedCigarettes * 1.0 / age;

       if(averageCigarettesPerYear >= 7300.0) {
           this.eventBus.fire(new PersonSmokesTooMuchEvent(this));
       }
  }

Of course this can be dangerous if somebody forgets to call setAge(int) where he should and sets age directly using this.age.

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Setters and getters are used to replace directly accessing member variables from external classes. if you use a setter and getter in accessing a property, you can include initialization, error checking, complex transformations, etc. Some examples:

private String x;

public void setX(String newX) {
    if (newX == null) {
        x = "";
    } else {
        x = newX;
    }
}

public String getX() {
    if (x == null) {
        return "";
    } else {
       return x;
    }
}

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Having accessor methods is preferred to accessing fields directly, because it controls how fields are accessed (may impose data checking etc) and fits with interfaces (interfaces can not requires fields to be present, only methods).

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Some benefits of using getters and setters (known as encapsulation or data-hiding):

(originally answered here)

1. The fields of a class can be made read-only (by only providing the getter) or write-only (by only providing the setter). This gives the class a total control of who gets to access/modify its fields.

Example:

class EncapsulationExample {
    private int readOnly = -1;  // this value can only be read, not altered
    private int writeOnly = 0;    // this value can only be changed, not viewed
    public int getReadOnly() {
        return readOnly;
    }
    public int setWriteOnly(int w) {
        writeOnly = w;
    }
}

2. The users of a class do not need to know how the class actually stores the data. This means data is separated and exists independently from the users thus allowing the code to be more easily modified and maintained. This allows the maintainers to make frequent changes like bug fixes, design and performance enhancements, all while not impacting users.

Furthermore, encapsulated resources are uniformly accessible to each user and have identical behavior independent of the user since this behavior is internally defined in the class.

Example (getting a value):

class EncapsulationExample {
    private int value;
    public int getValue() {     
        return value; // return the value
    }
}

Now what if I wanted to return twice the value instead? I can just alter my getter and all the code that is using my example doesn't need to change and will get twice the value:

class EncapsulationExample {
    private int value;
    public int getValue() {
        return value*2; // return twice the value
    }
}

3. Makes the code cleaner, more readable and easier to comprehend.

Here is an example:

No encapsulation:

class Box {
    int widthS; // width of the side
    int widthT; // width of the top
    // other stuff
}

// ...
Box b = new Box();
int w1 = b.widthS;  // Hm... what is widthS again? 
int w2 = b.widthT;  // Don't mistake the names. I should make sure I use the proper variable here!

With encapsulation:

class Box {
    private int widthS; // width of the side
    private int widthT; // width of the top
    public int getSideWidth() {
        return widthS;
    }
    public int getTopWIdth() {
        return widthT;
    }
    // other stuff
}

// ...
Box b = new Box();
int w1 = b.getSideWidth(); // Ok, this one gives me the width of the side
int w2 = b.getTopWidth(); // and this one gives me the width of the top. No confusion, whew!

Look how much more control you have on which information you are getting and how much clearer this is in the second example. Mind you, this example is trivial and in real-life the classes you would be dealing with a lot of resources being accessed by many different components. Thus, encapsulating the resources makes it clearer which ones we are accessing and in what way (getting or setting).

Here is good SO thread on this topic.

Here is good read on data encapsulation.

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The above answers summarize the role of getters and setters better than I could, however I did want to add that your code should ideally be structured to reduce the use of pure getters and setters, i.e. those without complex constructions, validation, and so forth, as they break encapsulation. This doesn't mean you can't ever use them (stivlo's answer shows an example of a good use of getters and setters), just try to minimize how often you use them.

The problem is that getters and setters can act as a workaround for direct access of private data. Private data is called private because it's not meant to be shared with other objects; it's meant as a representation of the object's state. Allowing other objects to access an object's private fields defeats the entire purpose of setting it private in the first place. Moreover, you introduce coupling for every getter or setter you write. Consider this, for example:

private String foo;

public void setFoo(String bar) {
    this.foo = bar;
}

What happens if, somewhere down the road, you decide you don't need foo anymore, or you want to make it an integer? Every object that uses the setFoo method now needs to be changed along with foo.

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just because the OOP rule: Data Hiding and Encapsulation. It is a very bad practice to declare a object's as public and change it on the fly in most situations. Also there are many other reasons , but the root is Encapsulation in OOP. and "buy a book or go read on Object Oriented Programming ", you will understand everything on this after you read any book on OOP.

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The benefits of get() set() methods are as follows ..

1. You can serialize you object easily.
2. You can create a persistent object from the containing class.
3. You can convert the properties to JSON easily.
4. In the DAO layer (Frameworks like Hibernate) you can directly save the object to DB.
5. Easy understanding of object oriented concept.
6. Needs in all design pattern except possibly in single tone pattern.
7. Security for properties protecting direct access.
8. Polymorphism, Encapsulation can be easily understood and implemented by this type of class.

Example:

private String personName;
private int personId;

    public void setPersonName(String name) throws Exception{
    if(!(name.equals("")||name=="")){
      this.personName = name;
    }
  }
  public String getPersonName(){
    return this.personName;
  }
  public void setPersonId(int id) throws Exception{
    this.personId = id;
  }
  public int getPersonId(){
    return this.personId;
  }

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Above answers all assume that the object in question is an object with behaviour. An advanced strategy in OOP is to separate data objects (that do zip, only have fields) and behaviour objects.

With data objects, it is perfectly fine to omit getters and instead have public fields. They usually don't have setters, since they most commonly are immutable - their fields are set via the constructors, and never again. Have a look at Bob Martin's Clean Code or Pryce and Freeman's Growing OO Software... for details.

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public class Person{

private int age;

public int getAge(){
     return age;
}

public void setAge(int age){
     this.age = age;
}
}

i think this is you want.. and this also called pojo

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this is the code for set method

public void setAge(int age){
  this.age = age;
}

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It looks like you trying to do something similar to C# if you want setAge create method
setAge(int age){ this.age = age;}

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I don't see a simple answer to the second question (why) here. So here goes.

Let's say you have a public field that gets used very often in your code. Whenever you decide you need to do something extra before you give or set this field you have a problem. You have to create a special getter and setter for this field and change your complete code from using the field directly to using the getter and setters.

Now imagine you are developing a library widely used by many people. When you need to make a change like the above and set direct access of the field to private the code of all the people using this field will break.

Using getters and setters is about future planning of the code, it makes it more flexible. Of course you can use public fields, especially for simple classes that just hold some data. But it's always a good idea to just make the field privately and code a get and set method for it.

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This answer is merged from another question.

Your getAge() method is called instance method in Java.

To invoke an instance method, you should have a object of the Class in which this method is defined.

For Example, If this method in a Class called Person, then

1. Create a Person object using new operator

    Person p = new Person();
   

2. To get the age of a Person object, use this method

   p.getAge()
   

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Although still a second year undergraduate student I will say my opinion. I believe that Java and private variables within your class are "RULES". Therefore because the variables in your class are private I think you use getters and setters to be able to define these variables outside the class.