IllegalArgumentException In Java

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import java.io.File;

public class JavaIllegalArgumentException {

	public static String createRelativePath(String parent, String filename) {

		if (parent == null)
			throw new IllegalArgumentException("path cannot be null!");

		return parent + File.separator + filename;
	}

	public static void main(String[] args) {
		// The following command will be successfully executed.
		System.out.println(JavaIllegalArgumentException.createRelativePath("dir1", "file1"));
		System.out.println();

		// The following command throws an IllegalArgumentException.
		System.out.println(JavaIllegalArgumentException.createRelativePath(null, "file1"));
	}
}
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