String variable interpolation Java

String building in Java confounds me. I abhore doing things like:

url += "u1=" + u1 + ";u2=" + u2 + ";u3=" + u3 + ";u4=" + u4 + ";";
url += "x=" + u1 + ";y=" + u2 + ";z=" + u3 + ";da1=" + u4 + ";";
url += "qty=1;cost=" + orderTotal + ";ord=" + orderId + "?";

Or, using StringBuilder, something like this:

	url.append("u1=");
	url.append(u1);
	url.append(";u2=");
	url.append(u2);
	url.append(";u3=");
	url.append(u3);
	url.append(";u4=");
	url.append(u4);
	url.append(";");
	url.append("x=");
	url.append(u1);
	url.append(";y=");
	url.append(u2);
	url.append(";z=");
	url.append(u3);
	url.append(";da1=");
	url.append(u4);
	url.append(";");
	url.append("qty=1;");
	url.append("cost=");
	url.append(orderTotal);
	url.append(";ord=");
	url.append(orderId);
	url.append("?");

SURELY I'm missing something. There has GOT to be a better way. Something like:

Instead of:

urlString += "u1=" + u1 + ";u2=" + u2 + ";u3=" + u3 + ";u4=" + u4 + ";";

do:

urlString += Interpolator("u1=%s;u2=%s;u3=%s;u4=%s;", u1, u2, u3, u4);

or:

urlStringBuilder.append(Interpolator("u1=%s;u2=%s;u3=%s;u4=%s;", u1, u2, u3, u4));

If you're using Java 5 or higher, you can use String.format:

urlString += String.format("u1=%s;u2=%s;u3=%s;u4=%s;", u1, u2, u3, u4);

See Formatter for details.

Note that there is no variable interpolation in Java. Variable interpolation is variable substitution with its value inside a string. An example in Ruby:

#!/usr/bin/ruby

age = 34
name = "William"

puts "#{name} is #{age} years old"

The Ruby interpreter automatically replaces variables with its values inside a string. The fact, that we are going to do interpolation is hinted by sigil characters. In Ruby, it is #{}. In Perl, it could be $, % or @. Java would only print such characters, it would not expand them.

Variable interpolation is not supported in Java. Instead of this, we have string formatting.

package com.zetcode;

public class StringFormatting 
{
    public static void main(String[] args) 
    {
        int age = 34;
        String name = "William";

        String output = String.format("%s is %d years old.", name, age);
    
        System.out.println(output);
    }
}

In Java, we build a new string using the String.format() method. The outcome is the same, but the methods are different.

See http://en.wikipedia.org/wiki/Variable_interpolation

Edit As of 2019, JEP 326 (Raw String Literals) was withdrawn and superseded by multiple JEPs eventually leading to JEP 378: Text Blocks delivered in Java 15.

>A text block is a multi-line string literal that avoids the need for most escape sequences, automatically formats the string in a predictable way, and gives the developer control over the format when desired.

However, still no string interpolation:

> Non-Goals: … > Text blocks do not directly support string interpolation. Interpolation may be considered in a future JEP. In the meantime, the new instance method String::formatted aids in situations where interpolation might be desired.

Just to add that there is also java.text.MessageFormat with the benefit of having numeric argument indexes.

Appending the 1st example from the documentation

int planet = 7;
String event = "a disturbance in the Force";

String result = MessageFormat.format(
    "At {1,time} on {1,date}, there was {2} on planet {0,number,integer}.",
    planet, new Date(), event);

Result:

At 12:30 PM on Jul 3, 2053, there was a disturbance in the Force on planet 7.

String.format() to the rescue!!

You can use Kotlin, the Super (cede of) Java for JVM, it has a nice way of interpolating strings like those of ES5, Ruby and Python.

class Client(val firstName: String, val lastName: String) {
    val fullName = "$firstName $lastName"
}