How to use java methods defined in another class/file

Mark your utility methods as public static. Package your classes containing those utility methods in a jar. Add/Refer that jar in your project, where you want to use the. Then in your code you can call them in a static way lke : MyUtilityClass.myUtilityMethod();

The best thing for this situation is to work in meaningful packages and make their jar
You can create a package like

/* File name : Animal.java */
package animals;

    interface Animal {
       public void eat();
       public void travel();
    }

Also on classes

package animals;

/* File name : MammalInt.java */
public class MammalInt implements Animal{

   public void eat(){
      System.out.println("Mammal eats");
   }

   public void travel(){
      System.out.println("Mammal travels");
   } 

   public int noOfLegs(){
      return 0;
   }

   public static void main(String args[]){
      MammalInt m = new MammalInt();
      m.eat();
      m.travel();
   }
} 

You can import them like
import animals.*; OR be more specific import animals.MammalInt;

Now you can make the jar file , import it in your project and use its method
You can eaisly do it by this command
jar cmf MyJar.jar Manifest.txt MyPackage/*.class
For more details about jar creation please see this
As a side note: Be carefull about visibility of members and functions while packaging it
Because there usage and accessibility matters a lot while we are using them

You could create separate java project with your util classes only and then create jar file and import into any another project.

Simply instantiate the class. Like your example, if you had a class MyMathMethods with the function primeFactor(number) then at other classes, simply instantiate it with something like private MyMathMethods myMathMethods;. Now, to call the function simply do myMathMethods.primeFactor(number); You may need to import its package as well.

False understanding of packages is any class defined within a package is visible to all other classes. Not true from my experience. If you have classes containing utility style methods you want to make available in another class? Simply declare a new instance of the class in the class you need the method in. Like... private MathUtilsClass mathUtilsClass = new MathUtilsClass(): Then any method you want to call from this class uses the new identifier, e.g. mathUtilsClass.greatFunction(); This is stupidly easy and should solve your problem.