Proxy404 Proxy404 - 1 year ago 86
Java Question

How to create a println/print method for a custom class

I'm working in Java on a project that requires me to make a few 'container' classes, if you will. Here is a simple version of one:

public class Pair{

Object KEY;
Object VALUE;

public Pair(Object k, Object v)
KEY = k;
VALUE = v;

public Object getKey()
{ return KEY; }
public Object getValue()
{ return VALUE; }

(Please note, this is severely simplified and I am using proper set/get methods in the final version.)

My question is this:

When calling the println method with an ArrayList as the parameter, for example:

ArrayList<String> arr = new ArrayList<String>();

Java automatically knows how to print each 'node' or element of the ArrayList correctly.

Is there a way to write a method to allow the println method to correctly print my Pair class?

Answer Source

You will need to override the toString method and return a string representation of what you want.

So for example:

public class Pair {

    Object key;
    Object value;

    public Pair(Object k, Object v)
        key = k;
        value = v;

    public Object getKey() { 
        return key; 

    public Object getValue() { 
        return value; 

    public String toString() {
        return "Key: " + getKey() + ", Value: " + getValue();

Than you can do the following:

List<Pair> pairs = new ArrayList<Pair>();
pairs.Add(new Pair("pair1key", "pair1value"));
pairs.Add(new Pair("pair2key", "pair2value"));

for (Pair p : pairs) {
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