System.getenv() in Java

System.getenv(): This method is available in java.lang.System class of Java.

Approach 1: When a method takes one argument.

Syntax:

String java.lang.System.getenv(String name)

This method takes one argument of type String as its parameter. This method gets the value of the specified environment variable. An environment variable is a system-dependent external named value.

Parameters: One parameter is required for this method.

name the name of the environment variable.

Returns: the string value of the variable, or null if the variable is not defined in the system environment.

Throws:

1. NullPointerException - if the name is null.

2. SecurityException - if a security manager exists and its checkPermissionmethod doesn't allow access to the environment variable name.

Approach 1.1: When no exceptions

Java

public class Systemgetenv {
    public static void main(String[] args) {

        String name = "os.name";
        System.out.println(System.getenv(name));
    }
}

Output:

null

Approach 1.1: NullPointerException 

Java

public class Systemgetenv {
    public static void main(String[] args) {

        String name = null;
        System.out.println(System.getenv(name));
    }
}

Output:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "s1" is null
	at java.base/java.lang.ProcessEnvironment$NameComparator.compare(ProcessEnvironment.java:202)
	at java.base/java.lang.ProcessEnvironment$NameComparator.compare(ProcessEnvironment.java:195)
	at java.base/java.util.TreeMap.getEntryUsingComparator(TreeMap.java:374)
	at java.base/java.util.TreeMap.getEntry(TreeMap.java:344)
	at java.base/java.util.TreeMap.get(TreeMap.java:279)
	at java.base/java.lang.ProcessEnvironment.getenv(ProcessEnvironment.java:279)
	at java.base/java.lang.System.getenv(System.java:1019)
	

Approach 2: When the method takes no arguments.

Syntax:

Map<String, String> java.lang.System.getenv()

This method returns an unmodifiable string map view of the current system environment. The environment is a system-dependent mapping from names to values which is passed from parent to child processes.

Note: If the system does not support environment variables, an empty map is returned.

Returns: the environment as a map of variable names to values.

Throws:

SecurityException - if a security manager exists and its checkPermissionmethod doesn't allow access to the process environment.

Java

public class Systemgetenv2 {
    public static void main(String[] args) {

        System.out.println("Number of values is " + System.getenv().size());
    }
}

Output:

Number of values is 43