System and Runtime Class

1. Introduction

In Java, the System and Runtime classes provide access to low-level system resources and JVM-level operations.

They belong to the java.lang package and are automatically available in every Java program.

• System → Provides access to standard input/output, environment variables, system properties, time, and garbage collection.
• Runtime → Represents the runtime environment of the JVM and allows interaction with system-level operations like memory and process execution.

These classes are powerful and commonly used in real-world applications.

2. System Class Overview

The System class provides:

• Standard input/output streams
• Access to system properties
• Access to environment variables
• Time measurement
• Manual garbage collection request
• Program termination

Important fact: All members of System are static.

You cannot create an object of System.

3. Standard Input and Output

3.1 System.out

Used for standard output.

System.out.println("Hello Java");

out is a static PrintStream object.

3.2 System.err

Used for error messages.

System.err.println("Error occurred");

3.3 System.in

Used for input.

import java.util.Scanner;

Scanner sc = new Scanner(System.in);
int num = sc.nextInt();

4. System Properties

System properties provide JVM and OS-related information.

System.out.println(System.getProperty("java.version"));
System.out.println(System.getProperty("os.name"));
System.out.println(System.getProperty("user.name"));

You can also set properties:

System.setProperty("myKey", "myValue");

5. Environment Variables

Environment variables come from the operating system.

String path = System.getenv("PATH");
System.out.println(path);

Difference:

• System properties → JVM-specific
• Environment variables → OS-level

6. Time Methods

6.1 currentTimeMillis()

Returns time in milliseconds since 1970.

long time = System.currentTimeMillis();
System.out.println(time);

Used in performance measurement.

6.2 nanoTime()

Returns high-precision time.

long start = System.nanoTime();
// code
long end = System.nanoTime();
System.out.println(end - start);

Better for measuring execution time.

7. System.exit()

Terminates the program.

System.exit(0);

• 0 → Normal termination
• Non-zero → Abnormal termination

Once called, program stops immediately.

8. System.gc()

Requests garbage collection.

System.gc();

Important:

• It only suggests GC.
• JVM decides whether to run it or not.

9. Runtime Class Overview

The Runtime class represents the running JVM.

Unlike System, you get its instance using:

Runtime rt = Runtime.getRuntime();

You cannot create Runtime object directly.

10. Memory Information Using Runtime

You can check memory details:

Runtime rt = Runtime.getRuntime();

System.out.println("Available Processors: " + rt.availableProcessors());
System.out.println("Free Memory: " + rt.freeMemory());
System.out.println("Total Memory: " + rt.totalMemory());
System.out.println("Max Memory: " + rt.maxMemory());

These are useful for performance tuning.

11. Executing External Programs

You can execute OS commands:

Runtime rt = Runtime.getRuntime();
rt.exec("notepad");

On Linux:

rt.exec("gedit");

Important: This starts a new system process.

12. Runtime vs System

13. Practical Example: Measuring Execution Time

long start = System.nanoTime();

for(int i = 0; i < 1000000; i++) {
    int x = i * i;
}

long end = System.nanoTime();
System.out.println("Time Taken: " + (end - start));

This is commonly used in performance analysis.

14. Summary

• System provides static utilities for I/O, properties, time, and program control.
• Runtime represents the JVM runtime environment.
• Use System for general system-level tasks.
• Use Runtime for memory management and executing external processes.
• Both belong to java.lang and are automatically available.
• They are powerful and should be used carefully.

Written By: Shiva Srivastava