WITH statement in Java

问题

In VB.NET there is the WITH command that lets you omit an object name and only access the methods and properties needed. For example:

With foo
   .bar()
   .reset(true)
   myVar = .getName()
End With

Is there any such syntax within Java?

Thanks!

回答1:

No. The best you can do, when the expression is overly long, is to assign it to a local variable with a short name, and use {...} to create a scope:

{
   TypeOfFoo it = foo; // foo could be any lengthy expression
   it.bar();
   it.reset(true);
   myvar = it.getName();
}

回答2:

Perhaps the closest way of doing that in Java is the double brace idiom, during construction.

Foo foo = new Foo() {{
    bar();
    reset(true);
    myVar = getName(); // Note though outer local variables must be final.
}};

Alternatively, methods that return this can be chained:

myName =
    foo
        .bar()
        .reset(true)
        .getName();

where bar and reset methods return this.

However, wanting to do this tends to indicate that the object does not have rich enough behaviour. Try refactoring into the called class. Perhaps there is more than one class trying to get out.

回答3:

You can get quite close using Java 8 lambdas, with a drawback of not being able to modify local variables.

Declare this method:

static <T> void with(T obj, Consumer<T> c) {
    c.accept(obj);
}

So you can use:

Window fooBarWindow = new Window(null);

String mcHammer = "Can't Touch This";

with(fooBarWindow, w -> {
     w.setAlwaysOnTop(true);
     w.setBackground(Color.yellow);
     w.setLocation(300, 300);

     w.setTitle(mcHammer); // can read local variables
     //mcHammer = "Stop!"; // won't compile - can't modify local variables
});

This is also possible using an anonymous class, but not as clean.

回答4:

Some objects allow you to "chain" method invocations, which approaches the syntax you like. For example, often a builder class will return itself from methods so you can do something like this:

MyObject b = new MyBuilder().setFoo(5).setBar(6).setBaz("garply!").build();

Each set... method returns this so you can chain the next invocation.

回答5:

Nope. Java has a policy of avoiding anything that might reduce verbosity.

Well, after writing this it just occurred to me that the closest thing might be static imports, e.g.

package a.b.c.d;
public class Foo {
   public static void bar() {
      ...
   }
}

and now you can do

package d.e.f;
import static a.b.c.d.Foo.*;

bar();

回答6:

The closest thing to this is static imports that will allow you to call static methods without explicitly specifying the class on which the method exists.

回答7:

As already said, you cannot really write the code as this in Java.

Just as a comment, if you are affraid of many copy/paste in the case you need to change the variable name, Eclipse allows you to rename all the references of the variable automatically:

Using ALT+SHIFT+R on the "foo" variable name, you can rename all at once to "myFoo" for instance:

Foo myFoo = new Foo();
myFoo.bar();
myFoo.reset(true);

回答8:

If you have hands on the implementation of Foo, you can use the fluent API concept.

Lets say you have that:

public class Foo {
    private String a;
    private String b;
    public void setA(String a) { this.a = a; }
    public void setB(String b) { this.b = b; }
    public String getName() { return this.a + " " + this.b; }
}

Then you could modify it to obtain this:

public class Foo {
    private String a;
    private String b;
    public Foo setA(String a) { this.a = a; return this; }
    public Foo setB(String b) { this.b = b; return this; }
    public String getName() { return this.a + " " + this.b; }
}

And your calling code could look like:

String name = new Foo().setA("foo")
                       .setB("bar")
                       .getName();

Enjoy!

来源：https://stackoverflow.com/questions/1494800/with-statement-in-java