How can I access the internal variable of a method in Java junit?

I have a function in which a random number is generated as shown below

public void method(){
     int number = random();
     // continue
}

My question is how can I access this variable without mocking random method?

I need this variable for my test scenarios.

One thing you can do if you really need this is refactor your code:

@RequiredArgsConstructor
public class MyClass {
 private<Supplier<Integer>> final random;
 
 public void method() {
  int number = random.get();
  // ...
 }
}

You can then inject it in the test

public class MyTest {
 @Test
 public void testMethod() {
  Supplier<Integer> supplier = () -> 3;
  MyClass sut = new MyClass(supplier);
  sut.method(); // random in method will be 3
 }
}

However, I assume you're doing something like this

public void method() {
 int random = random.get(),
 service.call(random);
}

then you can use an ArgumentCaptor.

@Test
public void testMethod() {
 Service service = mock(Service.class);
 MyClass sut = new MyClass(service);
 sut.method();
 ArgumentCaptor<Integer> captor = ArgumentCaptor.forClass(Integer.class);
 verify(service).call(captor.capture());
 // captor.value() now contains the value you're looking for
}