需要一些有关Java堆栈的帮助

Question

Have to say i'm a total newbie to java. recently i'm studying implementing data structures using java for an exam. while going through array based stacks i found below code.

    class StackX {

    private int maxSize;        // size of stack array
    private long[] stackArray;
    private int top;            // top of stack

   //-------------------------------------------------------------

    public StackX(int s)         // constructor
    {
    maxSize = s;             // set array size
    stackArray = new long[maxSize];  // create array
    top = -1;                // no items yet
    }


   //-------------------------------------------------------------
   public void push(long j)    // put item on top of stack
   {
   stackArray[++top] = j;     // increment top, insert item
   }

i have an idea about what first three lines for, but i'm troubled at understanding below lines.

     maxSize = s;             // set array size

     stackArray = new long[maxSize];  // create array

     top = -1;                // no items yet

Can someone explain why the use of [maxSize] in a array which is data type of long ? shouldn't it be numeric since the data type is long.

also why use long j on push(long j) there's no variable associated with j.

Help & comments are appreciated.

Answer 1

I suggest searching out some websites or books and start from the beginning, trying to just jump in and understand everything doesn't tend to work for most people, but in answer to your questions:

stackArray = new long[maxSize]; creates a new array of size 'maxsize' that can hold variables of the type long . 'maxsize' is of type int, which is a whole number, and the array can only have a whole number of elements in it.

long j is used because it means a method must be called at some point push(1.0) which calls the push method and passes in 1.0 as j . It then puts it in the array at position top +1.

Answer 2

Can someone explain why the use of [maxSize] in a array which is data type of long? shouldn't it be numeric since the data type is long.

maxSize is an integer variable, used to specify the size of the array which is created. So this statement:

stackArray = new long[maxSize];

creates an element with maxSize elements (eg 20) and then assigns a reference to that array to the stackArray variable.

It's not really clear what you were expecting otherwise, but I'd suggest reading the Java tutorial section on arrays .

also why use long j on push(long j) there's no variable associated with j.

This method declaration:

public void push(long j)

has a parameter called j . That means whenever you call the method you have to pass in a long value as the argument to the method. The initial value of the parameter for that method invocation is the same value that you pass in. From that point onwards, it's like a local variable for that method. Again, the Java tutorial on methods may help you.

Answer 3

Can someone explain why the use of [maxSize] in a array which is data type of long? shouldn't it be numeric since the data type is long.

Although your array might be declared as type long , the indexes are still int . No matter what is the type of your array, indexes are int . Look at the examples on this page for more information.

also why use long j on push(long j) there's no variable associated with j.

Look for the code that calls this method push() , you will find a variable associated with j . And since your array is of type long ,when you put in elements into this(push), they have to be of long type too. Hence, long j .