CODEWITHSUNDEEP
javastack
Sungam
Sungam

Reputation: 1734

Java Stack - Modify the last element in a stack

I have a stack of Integer in Java and I would like to know is there other way to increment "the last pushed item" by one without poped it first.

My current solution is 

Stack<Integer> stack = new Stack<Integer>();
stack.push(1);
stack.push(2);   // Increment 2 to 3
Integer last = stack.pop();
stack.push(last+1);

I tried the following but not working 

Integer last = stack.peek();
last+=1;

I thought it would work because last is a reference to the last element in the stack and it got incremented.

Upvotes: 1

Views: 5284

Answers (3)

Evgeniy Dorofeev
Evgeniy Dorofeev

Reputation: 136162

You need mutable long object for that, e.g. org.apache.commons.lang3.mutable.MutableLong or java.util.concurrent.atomic.AtomicLong. Then you can modify it

stack.peek().increment();

Upvotes: 1

ruakh
ruakh

Reputation: 183602

In Java, the Integer class is immutable, meaning that an Integer object can never be modified. This:

last += 1;

is really just shorthand for this:

last = Integer.valueOf(last.intValue() + 1);

That is, it actually creates (or retrieves) a different Integer instance, whose value is one greater than the value of the previous Integer instance.

If you want to be able to modify the object at the top of the stack, you'll need to put mutable objects on the stack. You can create your own IntWrapper or IntHolder or MutableInteger class for that. (But I don't particularly recommend this.)

Upvotes: 8

gefei
gefei

Reputation: 19856

with last+=1; you assign another object to last. Therefore, the stack is not manipulated.

Upvotes: 2

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