how to set default method argument values? [duplicate]

问题

Is it possible to set the default method parameter values in Java?

Example: If there is a method

public int doSomething(int arg1, int arg2)
{
//some logic here
    return 0;
}

is it possible to modify the given method in order to be able to call it with and without parameters?

example:

doSomething(param1, param2);
doSomething();

Thanks!

回答1:

You can accomplish this via method overloading.

public int doSomething(int arg1, int arg2)
{
        return 0;
}

public int doSomething()
{
        return doSomething(defaultValue0, defaultValue1);
}

By creating this parameterless method you are allowing the user to call the parameterfull method with the default arguments you supply within the implementation of the parameterless method. This is known as overloading the method.

回答2:

If your arguments are the same type you could use varargs:

public int something(int... args) {
    int a = 0;
    int b = 0;
    if (args.length > 0) {
      a = args[0];
    }
    if (args.length > 1) {
      b = args[1];
    }
    return a + b
}

but this way you lose the semantics of the individual arguments, or

have a method overloaded which relays the call to the parametered version

public int something() {
  return something(1, 2);
}

or if the method is part of some kind of initialization procedure, you could use the builder pattern instead:

class FoodBuilder {
   int saltAmount;
   int meatAmount;
   FoodBuilder setSaltAmount(int saltAmount) {
       this.saltAmount = saltAmount;
       return this;
   }
   FoodBuilder setMeatAmount(int meatAmount) {
       this.meatAmount = meatAmount;
       return this;
   }
   Food build() {
       return new Food(saltAmount, meatAmount);
   }
}

Food f = new FoodBuilder().setSaltAmount(10).build();
Food f2 = new FoodBuilder().setSaltAmount(10).setMeatAmount(5).build();

Then work with the Food object

int doSomething(Food f) {
    return f.getSaltAmount() + f.getMeatAmount();
}

The builder pattern allows you to add/remove parameters later on and you don't need to create new overloaded methods for them.

回答3:

No. Java doesn't support default parameters like C++. You need to define a different method:

public int doSomething()
{
   return doSomething(value1, value2);
}

回答4:

You can't declare default values for the parameters like C# (I believe) lets you, but you could simply just create an overload.

public int doSomething(int arg1, int arg2) {
    //some logic here
    return 0;
}

//overload supplies default values of 1 and 2
public int doSomething() {
    return doSomething(1, 2);
}

If you are going to do something like this please do everyone else who works with your code a favor and make sure you mention in Javadoc comments what the default values you are using are!

回答5:

It's not possible to default values in Java. My preferred way to deal with this is to overload the method so you might have something like:

public class MyClass
{
  public int doSomething(int arg1, int arg2)
  {
    ...
  }

  public int doSomething()
  {
    return doSomething(<default value>, <default value>);
  }
}

回答6:

You can overload the method with different parameters:

public int doSomething(int arg1, int arg2)
{
//some logic here
        return 0;
}

public int doSomething(
{
doSomething(0,0)
}

回答7:

Hopefully I am not misunderstanding this document, but if your using Java 1.8, in theory, you could accomplish something like this by defining a working implementation of a default method ("defender methods") within an interface that you implement.

interface DoInterface {
    void doNothing();
    public default void remove() {
       throw new UnsupportedOperationException("remove");
    }
    public default int doSomething( int arg1, int arg2) {
        val = arg1 + arg2 * arg1;
        log( "Value is: " + val );
        return val;
    }
}
class DoIt extends DoInterface {
    DoIt() {
        log("Called DoIt constructor.");
    }
    public int doSomething() {
        int val = doSomething( 0, 100 );
        return val;
    }
}

Then, call it either way:

DoIt d = new DoIt();
d.doSomething();
d.soSomething( 5, 45 );

来源：https://stackoverflow.com/questions/1038383/how-to-set-default-method-argument-values

标签

java

parameters

methods

default-value