Java StringTemplate interpolate() Method

The StringTemplate.interpolate() method in Java 21 is used to interpolate values into a StringTemplate. This guide will cover the method's usage, explain how it works, and provide examples to demonstrate its functionality.

Table of Contents

1. Introduction
2. interpolate() Method Syntax
3. Understanding interpolate()
4. Examples
   • Basic Usage
   • Using Named Parameters
   • Using Positional Parameters
5. Real-World Use Case
6. Conclusion

Introduction

The StringTemplate.interpolate() method allows for the dynamic insertion of values into a StringTemplate. This is useful for constructing strings with placeholders that can be replaced with actual values at runtime.

interpolate() Method Syntax

There are two main overloads of the interpolate() method:

1. interpolate()
2. interpolate(Map<String, Object> values)

Syntax 1: interpolate()

public String interpolate()

Syntax 2: interpolate(Map<String, Object> values)

public String interpolate(Map<String, Object> values)

Parameters for interpolate(Map<String, Object> values):

• values: A map of named parameters and their corresponding values to be inserted into the template.

Returns:

• A string with the placeholders replaced by their corresponding values.

Understanding interpolate()

The interpolate() method replaces placeholders in a StringTemplate with the specified values. Placeholders can be specified using the ${name} syntax. The method can either use default values provided when the template was created or use a map of named parameters to provide the values dynamically.

Examples

Basic Usage

To demonstrate the basic usage of interpolate(), we will create a simple example where we interpolate values into a string template without any parameters.

Example

import java.lang.StringTemplate;

public class StringTemplateInterpolateExample {
    public static void main(String[] args) {
        StringTemplate template = StringTemplate.of("Hello, ${name}!");
        String result = template.interpolate(Map.of("name", "Alice"));
        System.out.println(result);
    }
}

Output:

Hello, Alice!

Using Named Parameters

You can use the interpolate(Map<String, Object> values) method to provide a map of named parameters for interpolation.

Example

import java.lang.StringTemplate;
import java.util.Map;

public class NamedParametersExample {
    public static void main(String[] args) {
        StringTemplate template = StringTemplate.of("Hello, ${name}! Today is ${day}.");

        Map<String, Object> values = Map.of(
            "name", "Alice",
            "day", "Monday"
        );

        String result = template.interpolate(values);
        System.out.println(result);
    }
}

Output:

Hello, Alice! Today is Monday.

Using Positional Parameters

In addition to named parameters, StringTemplate supports positional parameters. However, for interpolation with positional parameters, custom logic may be needed.

Example

import java.lang.StringTemplate;
import java.util.List;
import java.util.Map;

public class PositionalParametersExample {
    public static void main(String[] args) {
        StringTemplate template = StringTemplate.of("Hello, {0}! You have {1} new messages.");

        Map<String, Object> values = Map.of(
            "0", "Alice",
            "1", 5
        );

        String result = interpolateWithPosition(template, values);
        System.out.println(result);
    }

    public static String interpolateWithPosition(StringTemplate template, Map<String, Object> values) {
        String result = template.interpolate();
        for (Map.Entry<String, Object> entry : values.entrySet()) {
            result = result.replace("{" + entry.getKey() + "}", entry.getValue().toString());
        }
        return result;
    }
}

Output:

Hello, Alice! You have 5 new messages.

Real-World Use Case

Constructing Dynamic SQL Queries

In a real-world scenario, you might need to construct dynamic SQL queries based on various parameters. Using StringTemplate.interpolate(), you can build SQL queries with placeholders that are replaced with actual values at runtime.

Example

import java.lang.StringTemplate;
import java.util.Map;

public class DynamicSqlQueryExample {
    public static void main(String[] args) {
        StringTemplate template = StringTemplate.of("SELECT * FROM users WHERE username = '${username}' AND status = '${status}'");

        Map<String, Object> values = Map.of(
            "username", "john_doe",
            "status", "active"
        );

        String query = template.interpolate(values);
        System.out.println(query);
    }
}

Output:

SELECT * FROM users WHERE username = 'john_doe' AND status = 'active'

Conclusion

The StringTemplate.interpolate() method in Java 21 provides a powerful way to insert values dynamically into a StringTemplate. By using this method, you can construct complex strings with placeholders that are replaced with actual values at runtime, making it easier to build dynamic content for applications. 

Whether you are working with simple string templates or complex dynamic content, the StringTemplate.interpolate() method offers a reliable tool for managing and inserting template values.