Concatenating chars to a String - Using StringBuilder's append()

Question

I was doing research on concatenating char primitive values to form a String and came across this post:

Concatenate chars to form String in java

I understand that the correct way of producing the final String value is to use the toString() method, how come that if I do not use this method, I still get the same output. I would have thought the following code would output the heap address of the object sb but it still prints 'ice'.

Thank you.

public class CharsToString { public static void main (String args[]) { char a, b, c; a = 'i'; b = 'c'; c = 'e'; StringBuilder sb = new StringBuilder(); sb.append(a); sb.append(b); sb.append(c); System.out.println(sb); } }

System.out.println(sb) is equal to System.out.println(sb.toString());. JVM will do it for you. You can create 2 simple classes: one with implemented toString, second without, and call println to bouth instances. — Nazar Sakharenko
– Nazar Sakharenko, Commented Aug 8, 2016 at 12:24

MrSmith42 · Accepted Answer · 2016-08-08 12:30:10Z

You need to look what System.out.println(sb); actually does. It calls in class java.io.PrintStream this method. (because a StringBuilder extends Object)

 public void println(Object x) { String s = String.valueOf(x); synchronized (this) { print(s); newLine(); } }

And, String.valueOf(x) calls the toString() method on your StringBuilder.

public static String valueOf(Object obj) { return (obj == null) ? "null" : obj.toString(); }

Thank you all for the input and MrSmith42 in particular for the extremely detailed answer.

studersi · Accepted Answer · 2016-08-08 12:22:11Z

1

The toString() method of the StringBuilder object is automatically called implicitly.

answered Aug 8, 2016 at 12:22

studersi

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Comments

SomeDude · Accepted Answer · 2016-08-08 12:24:35Z

If you check System.out.println(Object obj); System.out is a PrintStream and println( Object obj ) on the PrintStream calls String.valueOf( Object ),

The String.valueOf( Object ) calls toString() on the Object, so indirectly System.out.println( StringBuilder sb ) calls StringBuilder's toString(),

Now if your Object is other than StringBuilder, which doesn't have toString(), it will print getClass().getName() + "@" + Integer.toHexString(hashCode());

Hrabosch · Accepted Answer · 2016-08-08 12:25:22Z

This is because method toString() is called implicitly. Everytime, when you try to print object, it looks for toString() method. In this case, println method accept Object and call String.valueOf(Object o) which return String value of Object.

Michael Gantman · Accepted Answer · 2016-08-08 12:33:04Z

System.out is a PrintStream class. So you look at javadoc for println(Object) for that class you will get your answer: https://docs.oracle.com/javase/8/docs/api/java/io/PrintStream.html#println-java.lang.Object-. In short if you invoke method println with parameter that is not primitive and not a String it will print output of toString() method of your object. So in your case calling System.out.println(sb.toString()); and System.out.println(sb); would have identical result

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