CODEWITHSUNDEEP
javastack
Steven
Steven

Reputation: 51

Implement Stack using Java

I try to implement stack structure on Java and I have some problems on the pop method. Before pop, i want to check wether the stack is empty. The return says The local variable item may not have been initialized. When I remove if(!is.empty), it has no error. could someone tell me why? Thanks a lot.

public class ArrayStack {
private String[] s;
private int N=0;

    public ArrayStack(){
    s=new String[1];   //initialize the array;
}

private void reSize(int capacity){
    String[] copy=new String[capacity];
    for(int i=0;i<N;i++){
        copy[i]=s[i];
    }
    s=copy;
}

public boolean isEmpty(){
    return N==0;
}

public int stackSize(){
    return N;
}

public void push(String item){
    if(item!=null){
        if(N==s.length){
            reSize(s.length*2);
        }
    }
        s[N++]=item;
}

public String pop(){
    String item;
    if(isEmpty()){
        item=s[--N];
        s[N]=null;
    }
    if(N>0&&N==s.length/4){
        reSize(s.length/4);
    }
    return item;
}

}

Upvotes: 3

Views: 2869

Answers (2)

Jens Piegsa
Jens Piegsa

Reputation: 7495

Option 1, if you want to throws an exception when the stack is empty:

public String pop() {
    String item = s[--n];
    s[n] = null;
    if (n == (s.length / SHRINK_FACTOR)) {
        reSize(s.length / SHRINK_FACTOR);
    }
    return item;
}

Option 2, if you want to return null when the stack is empty:

public String pop() {
    String item = null;
    if (!isEmpty()) {
        item = s[--n];
        s[n] = null;
    }
    if (n == (s.length / SHRINK_FACTOR)) {
        reSize(s.length / SHRINK_FACTOR);
    }
    return item;
}

Some further notes:

  • before a variable can be used it must be given an initial value. Therefor if your condition is not met and item is returned without first giving it a value, the compiler will throw an error.

  • by convention, class attributes start with a small letter

  • instead of using magic numbers (4), introduce some constants in your class:

    private static final int SHRINK_FACTOR = 4;

  • consider writing unit tests to describe and ensure the expected behaviour:

ArrayStackTest.java:

import static org.junit.Assert.*;
import jdk.nashorn.internal.ir.annotations.Ignore;

import org.junit.Test;

public class ArrayStackTest {

    @Test
    public void testPushAndPop() {
        final ArrayStack stack = new ArrayStack();
        assertTrue(stack.isEmpty());
        stack.push("A");
        assertFalse(stack.isEmpty());
        stack.push("B");
        stack.push("C");
        assertEquals("C", stack.pop());
        assertEquals("B", stack.pop());
        assertEquals("A", stack.pop());
    }

    @Test(expected = Exception.class)
    public void testException() {
        final ArrayStack stack = new ArrayStack();
        stack.pop();
    }

    // remove @Ignore for option 2
    @Ignore
    @Test
    public void testPopReturnNull() {
        final ArrayStack stack = new ArrayStack();
        assertNull(stack.pop());
    }
}

Upvotes: 1

JoeC
JoeC

Reputation: 1850

You haven't initialize your local variable item

String item;

Change it to

String item = ""; or String item = NULL;

should resolve the issue

Upvotes: 0

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