Why java has a lot of duplicate methods?

I was playing with Java as I am planning to switch from C# to it for cross platform purposes. I have just noticed that it has a lot of methods that just do the same thing. And I just w开发者_如何学Goant to know why did they do that ?

An example, the Boolean class has two methods doing the same thing in addition to the constructor which does the same thing too.

Boolean b = new Boolean(true);
Boolean b = new Boolean("true");
Boolean b = Boolean.parseBoolean(true);
Boolean b = Boolean.parseBoolean("true");
Boolean b = Boolean.valueOf(true);
Boolean b = Boolean.valueOf("true");

And I can get the boolean value either by just calling the variable itself (b) or the method b.booleanValue(). Would anyone want to call a method getting the boolean value of a boolean although he can just call the variable itself ?

What is the point ?

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new Boolean(true) and Boolean.valueOf(true) return Boxed primitives. Real objects that can be used in collections etc. from primitive boolean values.

Boolean.parseBoolean("true") returns the primitive boolean value.

btw,

Boolean b = Boolean.parseBoolean(true);
Boolean b = Boolean.parseBoolean("true");

are really mistakes. you are creating a primitive boolean and then auto boxing to Boolean.

You should use valueOf(true) or valueOf("true") instead.

So the real use of these methods would be

Boolean b = new Boolean(true);   //really this should never be used **
Boolean b = new Boolean("true"); //really this should never be used **
boolean b = Boolean.parseBoolean(true);
boolean b = Boolean.parseBoolean("true");
Boolean b = Boolean.valueOf(true);
Boolean b = Boolean.valueOf("true");

** don't use this as you are just creating objects needlessly. Using valueOf allows for reusing existing Boolean objects. Since Booleans are immutable this is fine.

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1. Sometimes you need to parse string to primitive Boolean.parseBoolean(*String*)
2. Sometimes you need to parse String to Boolean Boolean.valueOf(*String*)
3. Sometimes you need not create new object. Better avoid using new
4. Sometimes you need the Boolean object instead of primitive Boolean.valueOf(*boolean*)

These are not same need.

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They are not really duplicate methods/constructors, if you notice difference between true and "true". true means primitive type boolean in Java but "true" means a java.lang.String object that has a value "true".

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you missed the funniest one

Boolean.getBoolean("true")

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What is the point ?

Well, the point is that some of those alternatives are useful, and some are old methods left over from the first version of Java.

(The original version of Java was released in a rush, and there were a few design mistakes / inconsistencies in the APIs. However, the overarching requirement to maintain backwards compatibility meant that it was impossible to correct them. In cases where the mistakes were positively harmful, the relevant methods have been marked as "deprecated" to warn programmers not to use them. In harmless cases like this where methods are simply redundant, things have been left unchanged.)

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Note that they are not the same; one of your lines:

Boolean b = Boolean.parseBoolean(true);

would give a syntax error (at least according to the Java 6 api).

Boolean.valueOf(true) and new Boolean(true) are different functions in that new Boolean(true) would create a new object and Boolean.valueOf(true) returns a stored Boolean object.

The signature of Boolean.parseBoolean returns a primitive boolean. Before Java 5 you needed Boolean.valueOf to convert it to an object form. After Java 5 the system will do that automatically, but (a) Java decided it wanted explicit forms of the autoboxing (and thus added Integer.valueOf and such) and (b) methods of Java are never deleted even when they become obsolete. In many cases that is a source of duplication itself (such as when they reorganized collections way back in Java 2 but old collection classes had methods added to match the new system leading to duplication).

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Boolean is a type that herits from Object, in opposit to b.booleanValue() it returns a primitive type boolean.

so the difference is that the first is an object and the second is a primitive type.

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You listed one that doesn't exist, and you incorrectly specified the return type of parseBoolean. The list is actually:

1. Boolean b = new Boolean(true);
2. Boolean b = new Boolean("true");
3. boolean b = Boolean.parseBoolean("true");
4. Boolean b = Boolean.valueOf(true);
5. Boolean b = Boolean.valueOf("true");

(4) is redundant with (1) and (5) is redundant with (2). Except two are constructors and two are methods. I suspect having that functionality from methods rather than from a constructor might be useful to something (factories?).

java.lang.Boolean