Can I write a java file which only contains method used for other classes

I am now developing some test method for different classes of the project. The whole project is too large so I want to re-construct it. I want to put all test method into one class or in one folder to make the test part separating from other functionality parts. Every test method only use attribute in corresponding classes,e.g.,Test_A.java---A.java, Test_B.java---B.java.

I am not familiar with design pattern of programs so I can't figure out a way to handle the problems after reading some notes. Could anyone suggest me a suitable design pattern to re-construct the test part? And it is better if you can provide the pattern with easy-understanding samples or sample links