Printing Java collections nicely (toString doesn't return pretty output)

Question

I wish to print a stack object as nicely as the Eclipse debugger does, ie [1,2,3] etc. printing it like so out = "outout:"+stack doesn't return this nice result.

Just to clarify. I'm talking about Java's builtin collections I can't override their toString.

How can get the nice printable version of the stack?

Answer 1

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

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

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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));    
}

}