Why java has a lot of duplicate methods?

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.

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".

you missed the funniest one

Boolean.getBoolean("true")

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.