String variable interpolation Java

Question

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));

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

String.format() to the rescue!!

Answer 5

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"
}