Printing Java Collections Nicely (toString Doesn't Return Pretty Output)

I wish to print a Stack<Integer> object as nicely as the Eclipse debugger does (i.e. [1,2,3...]) but printing it with out = "output:" + stack doesn't return this nice result.

Just to clarify, I'm talking about Java's built-in collection so I can't override its toString().

How can I get a nice printable version of the stack?

You could convert it to an array and then print that out with Arrays.toString(Object[]):

System.out.println(Arrays.toString(stack.toArray()));

String.join(",", yourIterable);

(Java 8)

With java 8 streams and collectors it can be done easily:

String format(Collection<?> c) {
  String s = c.stream().map(Object::toString).collect(Collectors.joining(","));
  return String.format("[%s]", s);
}

first we use map with Object::toString to create Collection<String> and then use joining collector to join every item in collection with , as delimiter.

The MapUtils class offered by the Apache Commons project offers a MapUtils.debugPrint method which will pretty print your map.

Guava looks like a good option:

Iterables.toString(myIterable)

System.out.println(Collection c) already print any type of collection in readable format. Only if collection contains user defined objects , then you need to implement toString() in user defined class to display content.

Implement toString() on the class.

I recommend the Apache Commons ToStringBuilder to make this easier. With it, you just have to write this sort of method:

public String toString() {
     return new ToStringBuilder(this).
       append("name", name).
       append("age", age).
       toString(); 
}

In order to get this sort of output:

> Person@7f54[name=Stephen,age=29]

There is also a reflective implementation.

I agree with the above comments about overriding toString() on your own classes (and about automating that process as much as possible).

For classes you didn't define, you could write a ToStringHelper class with an overloaded method for each library class you want to have handled to your own tastes:

public class ToStringHelper {
    //... instance configuration here (e.g. punctuation, etc.)
    public toString(List m) {
        // presentation of List content to your liking
    }
    public toString(Map m) {
        // presentation of Map content to your liking
    }
    public toString(Set m) {
        // presentation of Set content to your liking
    }
    //... etc.
}

EDIT: Responding to the comment by xukxpvfzflbbld, here's a possible implementation for the cases mentioned previously.

package com.so.demos;

import java.util.List;
import java.util.Map;
import java.util.Set;

public class ToStringHelper {

    private String separator;
    private String arrow;

    public ToStringHelper(String separator, String arrow) {
        this.separator = separator;
        this.arrow = arrow;
    }

   public String toString(List<?> l) {
        StringBuilder sb = new StringBuilder("(");
        String sep = "";
        for (Object object : l) {
            sb.append(sep).append(object.toString());
            sep = separator;
        }
        return sb.append(")").toString();
    }

    public String toString(Map<?,?> m) {
        StringBuilder sb = new StringBuilder("[");
        String sep = "";
        for (Object object : m.keySet()) {
            sb.append(sep)
              .append(object.toString())
              .append(arrow)
              .append(m.get(object).toString());
            sep = separator;
        }
        return sb.append("]").toString();
    }

    public String toString(Set<?> s) {
        StringBuilder sb = new StringBuilder("{");
        String sep = "";
        for (Object object : s) {
            sb.append(sep).append(object.toString());
            sep = separator;
        }
        return sb.append("}").toString();
    }

}

This isn't a full-blown implementation, but just a starter.

You can use the "Objects" class from JAVA (which is available since 1.7)

Collection<String> myCollection = Arrays.asList("1273","123","876","897");
Objects.toString(myCollection);

Output: 1273, 123, 876, 897

Another possibility is to use the "MoreObjects" class from Google Guave, which provides many of useful helper functions:

MoreObjects.toStringHelper(this).add("NameOfYourObject", myCollection).toString());

Output: NameOfYourObject=[1273, 123, 876, 897]

Guava docs

In Java8

//will prints each element line by line
stack.forEach(System.out::println);

or

//to print with commas
stack.forEach(
    (ele) -> {
        System.out.print(ele + ",");
    }
);

With Apache Commons 3, you want to call

StringUtils.join(myCollection, ",")

most collections have a useful toString() in java these days (Java7/8). So there is no need to do stream operations to concatenate what you need, just override toString of your value class in the collection and you get what you need.

both AbstractMap and AbstractCollection implement toString() by calling toString per element.

below is a testclass to show behaviour.

import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import java.util.HashMap;

public class ToString {
  static class Foo {
    int i;
    public Foo(int i) { this.i=i; }
    @Override
    public String toString() {
        return "{ i: " + i + " }";
    }
  }
  public static void main(String[] args) {
    List<Foo> foo = new ArrayList<>();
    foo.add(new Foo(10));
    foo.add(new Foo(12));
    foo.add(new Foo(13));
    foo.add(new Foo(14));
    System.out.println(foo.toString());
    // prints: [{ i: 10 }, { i: 12 }, { i: 13 }, { i: 14 }]

    Map<Integer, Foo> foo2 = new HashMap<>();
    foo2.put(10, new Foo(10));
    foo2.put(12, new Foo(12));
    foo2.put(13, new Foo(13));
    foo2.put(14, new Foo(14));
    System.out.println(foo2.toString());
    // prints: {10={ i: 10 }, 12={ i: 12 }, 13={ i: 13 }, 14={ i: 14 }}
  }
}

Update Java 14 (Mar 2020)

Records are now a preview feature not requiring you to override toString() if your class only holds data. Records implement a data contract, giving public readonly access to it's fields and implementing default functions for your convience, like comparison, toString and hashcode.

so once could implement Foo as follows with the behavior:

import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import java.util.HashMap;

public class ToString {
  static record Foo(int i) { }
  
  public static void main(String[] args) {
    Foo f = new Foo(10);
    System.out.println(f.toString());
    // prints: Foo[i=10]

    List<Foo> foo = new ArrayList<>();
    foo.add(new Foo(10));
    foo.add(new Foo(12));
    foo.add(new Foo(13));
    foo.add(new Foo(14));
    System.out.println(foo.toString());
    // prints: [Foo[i=10], Foo[i=12], Foo[i=13], Foo[i=14]]

    Map<Integer, Foo> foo2 = new HashMap<>();
    foo2.put(10, new Foo(10));
    foo2.put(12, new Foo(12));
    foo2.put(13, new Foo(13));
    foo2.put(14, new Foo(14));
    System.out.println(foo2.toString());
    // prints: {10=Foo[i=10], 12=Foo[i=12], 13=Foo[i=13], 14=Foo[i=14]}
  }
}

If this is your own collection class rather than a built in one, you need to override its toString method. Eclipse calls that function for any objects for which it does not have a hard-wired formatting.

Just Modified the previous example to print even collection containing user defined objects.

public class ToStringHelper {
    
    private  static String separator = "\n";
    
    public ToStringHelper(String seperator) {
        super();
        ToStringHelper.separator = seperator;
    	
    }
    
    public  static String toString(List<?> l) {
        StringBuilder sb = new StringBuilder();
        String sep = "";
        for (Object object : l) {
            String v = ToStringBuilder.reflectionToString(object);
            int start = v.indexOf("[");
            int end = v.indexOf("]");
            String st =  v.substring(start,end+1);
            sb.append(sep).append(st);
            sep = separator;
        }
        return sb.toString();
    }
    
    public static String toString(Map<?,?> m) {
    	StringBuilder sb = new StringBuilder();
        String sep = "";
        for (Object object : m.keySet()) {
            String v = ToStringBuilder.reflectionToString(m.get(object));
            int start = v.indexOf("[");
            int end = v.indexOf("]");
            String st =  v.substring(start,end+1);
            sb.append(sep).append(st);
            sep = separator;
        }
        return sb.toString();
    }
    
    public static String toString(Set<?> s) {
        StringBuilder sb = new StringBuilder();
        String sep = "";
        for (Object object : s) {
            String v = ToStringBuilder.reflectionToString(object);
            int start = v.indexOf("[");
            int end = v.indexOf("]");
            String st =  v.substring(start,end+1);
            sb.append(sep).append(st);
            sep = separator;
        }
        return sb.toString();
    }
    
    public static void print(List<?> l) {
    	System.out.println(toString(l));	
    }
    public static void print(Map<?,?> m) {
    	System.out.println(toString(m));	
    }
    public static void print(Set<?> s) {
    	System.out.println(toString(s));	
    }
    
}

Be careful when calling Sop on Collection, it can throw ConcurrentModification Exception. Because internally toString method of each Collection internally calls Iterator over the Collection.

You can try using

org.apache.commons.lang3.builder.ToStringBuilder.reflectionToString(yourCollection);

There are two ways you could simplify your work.

1. import Gson library.
2. use Lombok.

Both of them help you create String from object instance. Gson will parse your object, lombok will override your class object toString method.

I put an example about Gson prettyPrint, I create helper class to print object and collection of objects. If you are using lombok, you could mark your class as @ToString and print your object directly.

@Scope(value = "prototype")
@Component
public class DebugPrint<T> {
   public String PrettyPrint(T obj){
      Gson gson = new GsonBuilder().setPrettyPrinting().create();
      return gson.toJson(obj);
   }
   public String PrettyPrint(Collection<T> list){
      Gson gson = new GsonBuilder().setPrettyPrinting().create();
      return list.stream().map(gson::toJson).collect(Collectors.joining(","));
   }

}

JSON

An alternative Solution could be converting your collection in the JSON format and print the Json-String. The advantage is a well formatted and readable Object-String without a need of implementing the toString().

Example using Google's Gson:

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

...

    printJsonString(stack);

...
public static void printJsonString(Object o) {
	GsonBuilder gsonBuilder = new GsonBuilder();
	/*
	 * Some options for GsonBuilder like setting dateformat or pretty printing
	 */
	Gson gson = gsonBuilder.create();
	String json= gson.toJson(o);
	System.out.println(json);
}

Newer JDK already has AbstractCollection.toString() implemented, and Stack extends AbstractCollection so you just have to call toString() on your collection:

    public abstract class AbstractCollection<E> implements Collection<E> {
    ...
    public String toString() {
        Iterator<E> it = iterator();
        if (! it.hasNext())
            return "[]";
    
        StringBuilder sb = new StringBuilder();
        sb.append('[');
        for (;;) {
            E e = it.next();
            sb.append(e == this ? "(this Collection)" : e);
            if (! it.hasNext())
                return sb.append(']').toString();
            sb.append(',').append(' ');
        }
    }

Should work for any collection except Map, but it's easy to support, too. Modify code to pass these 3 chars as arguments if needed.

static <T> String seqToString(Iterable<T> items) {
    StringBuilder sb = new StringBuilder();
    sb.append('[');
    boolean needSeparator = false;
    for (T x : items) {
        if (needSeparator)
            sb.append(' ');
        sb.append(x.toString());
        needSeparator = true;
    }
    sb.append(']');
    return sb.toString();
}