CODEWITHSUNDEEP
javalistarraylist
Kipras
Kipras

Reputation: 163

How do elements in an ArrayList work?

If I add an object to an ArrayList in Java, I add a reference to it, right? So assume this situation. I create an object called obj and set it to a value. I then loop a certain amount of times and each time I modify obj a little (like change its position) and then add it to the ArrayList. After each sequence of the loop I reset obj to the default value so the next sequence will have a fresh obj to modify. Assuming I had been passing references of obj all the time, everything in the ArrayList will be set to the default value also? From testing I see that the values don't change and are all different, which I find strange. Could somebody elaborate?

Block b = block.clone();

    for(int x = 0; x < amountX; x++){
        Vector3f offset = new Vector3f(0, 0, 0);
        offset.x = x * block.getScale().x;

        for(int y = 0; y < amountY; y++){
            offset.y = y * block.getScale().y;

            for(int z = 0; z < amountZ; z++){
                offset.z = z * block.getScale().z;
                b.move(offset);
                addBlocks(b);
                b = block.clone();
            }
        }
    }

    for(Block a : blocks)
        System.out.println(a.getPosition());

addBlocks adds a block to the ArrayList. The last loop prints out all different positions. Block.clone() just creates a new block with the same parameters.

Upvotes: 4

Views: 148

Answers (1)

Ross Drew
Ross Drew

Reputation: 8246

 Object o = new Object();

At this point you are not modifying the object any more, you are modifying the reference. i.e. creating a new object and pointing this variable at it. 

 do
 {
   Object o = new Object(); // set o to point to a new Object
   arrayList.add(o); //add a reference to the new object to the ArrayList
 }while(true)

the second time through this loop, the first object created in memory at pos1 say is referenced by arrayList[1]. But you then change where object o is pointing in memory by creating a new one (at pos2) and pointing to it with o. You now have two object in memory, one referenced at [1] in the arrayList and one referenced at object o.

If you modify the object and not the reference, you will get the behaviour you talk about, e.g.

  do
 {
   g.setName("Steven");
   arrayList.add(o);
 }while(true)

Upvotes: 3

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