Without more details as to what the question is exactly asking, I am going to answer the title of the question,
Create an Array
:
String[] myArray = new String[2];
int[] intArray = new int[2];
// or can be declared as follows
String[] myArray = {"this", "is", "my", "array"};
int[] intArray = {1,2,3,4};
Create an ArrayList
:
ArrayList<String> myList = new ArrayList<String>();
myList.add("Hello");
myList.add("World");
ArrayList<Integer> myNum = new ArrayList<Integer>();
myNum.add(1);
myNum.add(2);
This means, create an ArrayList
of String
and Integer
objects. You cannot use int
because thats a primitive data types, see the link for a list of primitive data types.
Create a Stack
:
Stack myStack = new Stack();
// add any type of elements (String, int, etc..)
myStack.push("Hello");
myStack.push(1);
Create an Queue
: (using LinkedList)
Queue<String> myQueue = new LinkedList<String>();
Queue<Integer> myNumbers = new LinkedList<Integer>();
myQueue.add("Hello");
myQueue.add("World");
myNumbers.add(1);
myNumbers.add(2);
Same thing as an ArrayList
, this declaration means create an Queue
of String
and Integer
objects.
Update:
In response to your comment from the other given answer,
i am pretty confused now, why are using string. and what does
<String>
means
We are using String
only as a pure example, but you can add any other object, but the main point is that you use an object not a primitive type. Each primitive data type has their own primitive wrapper class, see link for list of primitive data type’s wrapper class.
I have posted some links to explain the difference between the two, but here are a list of primitive types
byte
short
char
int
long
boolean
double
float
Which means, you are not allowed to make an ArrayList
of integer’s like so:
ArrayList<int> numbers = new ArrayList<int>();
^ should be an object, int is not an object, but Integer is!
ArrayList<Integer> numbers = new ArrayList<Integer>();
^ perfectly valid
Also, you can use your own objects, here is my Monster
object I created,
public class Monster {
String name = null;
String location = null;
int age = 0;
public Monster(String name, String loc, int age) {
this.name = name;
this.loc = location;
this.age = age;
}
public void printDetails() {
System.out.println(name + " is from " + location +
" and is " + age + " old.");
}
}
Here we have a Monster
object, but now in our Main.java
class we want to keep a record of all our Monster
‘s that we create, so let’s add them to an ArrayList
public class Main {
ArrayList<Monster> myMonsters = new ArrayList<Monster>();
public Main() {
Monster yetti = new Monster("Yetti", "The Mountains", 77);
Monster lochness = new Monster("Lochness Monster", "Scotland", 20);
myMonsters.add(yetti); // <-- added Yetti to our list
myMonsters.add(lochness); // <--added Lochness to our list
for (Monster m : myMonsters) {
m.printDetails();
}
}
public static void main(String[] args) {
new Main();
}
}
(I helped my girlfriend’s brother with a Java game, and he had to do something along those lines as well, but I hope the example was well demonstrated)