I already have an infix to postfix converter, now I want to get the answer from the postfix notation.

Here is my code to evaluate a given postfix notation and (hopefully) get the answer:

public class testEVAL {

    private StackArray SA;
    private String input;
    private int postfixEVAL;
    
    //start constructor
    public testEVAL(String input){
        
        SA = new StackArray();
        this.input = input;
        //postfixEVAL =;
        
    }//end constructor

    private int obj2intCONVERT(Object obj){
        return Integer.parseInt(obj.toString());
    }// converting popped Objects into Integers

    //start operation methods
    private int add(int a,int b){        
        return a+b;
    }
    private int sub(int a,int b){
        return a-b;
    }
    private int mul(int a,int b){
        return a*b;
    }
    private int div(int a,int b){
        return a/b;
    }//end operation methods

    public int postEVALUATED(){
        postEVAL();
        return postfixEVAL;
    }//returns the answer of the given postfix notation

    private boolean isDIGIT(char ch){
        return Character.isDigit(ch);
    }//is digit sort of override

    private int operatorCHK(char ch){
        int ret=-1;
        switch(ch){
            case '+':
                ret=1;
                break;
            case '-':
                ret=2;
                break;
            case '*':
                ret=3;
                break;
            case '/':
                ret=4;
                break;
        }

        return ret;
    }//returns 1 for addition, 2 for subtraction, 3 for multiplication and 4 for division

    private void postEVAL(){

        int numA,numB,numANS=0;

        for(int i = 0; i < input.length();++i){
            if(input.charAt(i) != '+' && input.charAt(i) != '-' && input.charAt(i) != '*' && input.charAt(i) != '/'){
                SA.push(input.charAt(i));
            }
            else if(!SA.isEmpty()){
                    numB = SA.intPOP();
                    numA = SA.intPOP();
                    switch(operatorCHK(input.charAt(i))){
                    case '1':
                        numANS=add(numA,numB);
                        SA.push(numANS);
                        break;
                    case '2':
                        numANS=sub(numA,numB);
                        SA.push(numANS);
                        break;
                    case '3':
                        numANS=mul(numA,numB);
                        SA.push(numANS);
                        break;
                    case '4':
                        numANS=div(numA,numB);
                        SA.push(numANS);
                        break;
                }//end switch                                
            }//end ifs
            if(i==input.length()-1){
                postfixEVAL = numANS;
            }
        }//end for

    }
}

My problem is, the output returns 0 (numANS "should" have the answer by the last iteration in the for loop in the postEVAL method)...

Any help would be much appreciated!

My guess is that there won't be many people wantinmg to go thru your code line by line debugging it (me included), so here's the next best thing - some help for you to debug it yourself.
What I don't see in the code is a whole load of
System.out.println(key info about what's going on);
at all the important stages of your code.
Without that kind of info, all you know is "it don't work". Put in the prints to see if (a) the right branches are being taken and (b) the data values involved are exactly what you expect them to be.
eg at line 59 print "ret" to see if it's right.
at line 73 print numA and numB, etc.
Put enough in and it will suddenly become blindingly obvious to you where its going wrong. When you've fixed it don't delete the prints, just comment them out - they'll come in useful again next time it stops working.

It also helps in debugging code to have a main() that calls the code to test with a hardcoded value being passed to the code to be tested and that compares the results with a known correct answer.

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