How to solve"error: could not find or load main class"

First of all, I noticed that you are trying to use nested functions in Java, sadly, they are not at all supported in the Java language.

public Test1() {
public static void test (int num1,int num2,int num3) {

You need to terminate the constructor, and then start another function. There is a concept of lambda functions[^], but that is something else.
Another thing that I saw in your code was that you are not following the standards for main function in Java programs. If you read the standard for Java's main function[^], you will find out that the function has to be declared as,

public static void main(String[] args) { }

This is the standard that your program must follow. Try the following code instead,

public class Test1 {
   public Test1() {
   }

   // Add this
   public static void main(String[] args) {
      // Call your function
      test(0, 0, 0); // Because you are setting values later. 
   }

   public static void test (int num1,int num2,int num3) {
      num1= 20 ;
      num2= 22 ;
      num3= num1+num2 ;
      System.out.println(num3);
   }
}

This should work, unless your Java compiler is expecting another class to be the main class for your program. Secondly, start learning Java from here, Trail: Learning the Java Language (The Java™ Tutorials)[^], I really enjoyed learning the Java basics from here and apply them in NetBeans. NetBeans is a simple IDE for Java beginners, Eclipse can be hard sometimes.

For more, please look:
Lesson: A Closer Look at the "Hello World!" Application (The Java™ Tutorials > Getting Started)[^]