native methods
- Java course
- Expert
- Lesson 25/46
- 5 min
Learn native method declaration with the native keyword.
The native keyword
The native keyword marks a method as implemented in a language other than Java (usually C or C++). Such a method has no body; it ends with a semicolon. At call time the JVM bridges to the native code through JNI (Java Native Interface).
native methods are used for operating system calls, legacy C libraries or hardware access. In modern Java 21 the FFM API is preferred for many cases; still, JNI remains common.
Declaring a native method
A native method is declared like this: native returnType methodName(params);. There is no body, only a semicolon. native can be combined with static. The method must belong to a class; Java has no free functions.
Example: class NativeOps { native int sum(int a, int b); }. Before this method is called, the matching native library must be loaded with System.loadLibrary; otherwise an UnsatisfiedLinkError is thrown.
Practice Declare a native method and print a marker
Define class NativeOps with a native int sum(int a, int b); method (no body). In Main's main method print System.out.println("native declared");. Do not call the native method; only declare it.
Starter code
// Define the NativeOps class: native int sum(int a, int b)
// Define the Main class and print native declared in mainSample solution
class NativeOps {
native int sum(int a, int b);
}
public class Main {
public static void main(String[] args) {
System.out.println("native declared");
}
}Expected output
native declared
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the defining trait of native methods?
- They appear only in abstract classes
- They have no body, implemented in another language Correct answer
- They must be static
- They are auto-tested
Explanation: A native method has no body; its implementation is written in C or C++.
2 Complete the native method declaration
class NativeOps {
native ____(int a, int b);
}class NativeOps {
native int sum(int a, int b);
}Explanation: The declaration native int sum(int a, int b); is complete.
3 Match the JNI concepts
| Term | Meaning |
|---|---|
native |
Bodyless method marker |
System.loadLibrary |
Loads native library |
JNIEnv |
C-side interface |
javac -h |
Generates header |