方法创建新实例

Question

When a method returns an object, does that method automatically create a new instance of that object in memory?

For example:

In Java BigInteger class , I use the add method where num1 , and num2 , lets say, are BigIntegers .

num1.add(num2);

Then I reference it by the same type as in:

BigInteger a = num1.add(num2);

This works and gets the correct data. So the method will create a new instance of that object on the stack?

Just making sure I'm correct about my assumption.

Thanks.

Answer 1

bigInteger.add() does return a new instance, but not all methods do.

What usually returns new object instances that you may run into regularly are:

• Constructors
• Factorys/Builders
• methods operating on immutable objects

BigInteger 's add method says that it returns a BigInteger , doesn't say a new BigInteger , but given that the docs also state that BigInteger is immutable , then you can know it's not returning the this object with a modification, so chances are that it's a new object (I suppose it's possible to be some cached object already representing that state, but even if it was, your usage of BigInteger wouldn't change.).

Answer 2

It depends.

With your BigInteger example, it is returning a new instance of the object.

However with something like StringBuilder, you can do

stringBuilder.append("this").append("is").append("a").append("string");

and each .append(String) is returning the same original object, it just allows you to chain the calls together.