Java: Getting the properties of a class to construct a string representation

Question

Let's say I have a class like this (and also further assume that all the private variables:

public class Item {     private String _id = null;     private String _name = null;     private String _description = null;          ... } 

Now, if I want to build a toString() representation of this class, I would do something like this inside the Item class:

@Override public String toString() {     return (_id + " " + _name + " " + _description); } 

But what if I have say 15 private variables inside the class? Do I have to write the name of each and every variable like this?

Ideally, I would like to get over with the task by iterating through the list of private variables of this class and construct the string representation:

@Override public String toString() {     ArrayList<String> members = getClass().getMembers(); //Some method like this     String string = "";     for(...)         string += members[i] + " "; } 

Or perhaps a toJSON method, I would still need access to the names of these variables. Any suggestions?

Answer 1

You could do:

@Override public String toString() {   StringBuilder sb = new StringBuilder();   sb.append(getClass().getName());   sb.append(": ");   for (Field f : getClass().getDeclaredFields()) {     sb.append(f.getName());     sb.append("=");     sb.append(f.get(this));     sb.append(", ");   }   return sb.toString(); } 

Don't use string concatenation to construct an end result from 15 data members, particularly if the toString() will be called a lot. The memory fragmentation and overhead could be really high. Use StringBuilder for constructing large dynamic strings.

I usually get my IDE (IntelliJ) to simply generate toString() methods for me rather than using reflection for this.

Another interesting approach is to use the @ToString annotation from Project Lombok:

import lombok.ToString;  @ToString(excludes="id") public class ToStringExample {   private static final int STATIC_VAR = 10;   private String name;   private Shape shape = new Square(5, 10);   private String[] tags;   private int id;    @ToString(callSuper=true, includeFieldNames=true)   public static class Square extends Shape {     private final int width, height;      public Square(int width, int height) {       this.width = width;       this.height = height;     }   } } 

I find this much more preferable to, say, Jakarta Commons toString builders because this approach is far more configurable and it's also built at compile-time not run-time.

Answer 2

Check this API org.apache.commons.lang.builder.ToStringBuilder, it provides multiple ways to create toString usinf reflection or without reflection. Take a look at other subclasses as well.