Static predefined methods & the static keyword (course memo + handout)

Java.

The static keyword

static marks an element as belonging to the class itself, not to individual object instances. A static field is created once and shared across every instance of the class; static can apply to fields and methods (among other things).

public class MrMeeseeks {
    private static int numberOfMeeseeks = 0;
    private String task;

    public MrMeeseeks(String task) {
        this.task = task;
        numberOfMeeseeks++;
    }

    public static int getNumberOfMeeseeks() {
        return numberOfMeeseeks;
    }

    public String doTask() {
        return "I will " + task;
    }
}

• numberOfMeeseeks is shared by all MrMeeseeks instances ever created.
• getNumberOfMeeseeks() is static and callable without any instance: MrMeeseeks.getNumberOfMeeseeks().
• doTask() is not static — it can only be called on a specific instance (meeseeks1.doTask()), since it depends on that instance's own task.

class MrMeeseeksTest {
    @Test
    void testNumberOfMeeseeks() {
        Assertions.assertEquals(0, MrMeeseeks.getNumberOfMeeseeks());

        MrMeeseeks meeseeks1 = new MrMeeseeks("do the laundry");
        Assertions.assertEquals(1, MrMeeseeks.getNumberOfMeeseeks());
        Assertions.assertEquals("I will do the laundry", meeseeks1.doTask());

        MrMeeseeks meeseeks2 = new MrMeeseeks("do the dishes");
        Assertions.assertEquals(2, MrMeeseeks.getNumberOfMeeseeks());
    }
}

Static members are accessed via the class name, not an instance: MrMeeseeks.getNumberOfMeeseeks(). Course guidance: avoid using static in your own classes until you have more experience — it's easy to misuse (shared mutable state across every instance, harder-to-test code).

Collections static methods

Collections.reverse(collection), Collections.sort(collection), Collections.shuffle(collection).

Given names = [Lisa, Mona, Sam, Anton]:

Call	Result
Collections.reverse(names)	[Anton, Sam, Mona, Lisa]
Collections.sort(names)	[Anton, Lisa, Mona, Sam] (via each element's compareTo)
Collections.shuffle(names)	randomly rearranged

These mutate the list in place rather than returning a new one — so the list passed in must be mutable (e.g. an ArrayList, not one made with List.of(...)). To get a mutable copy of an immutable list: new ArrayList<>(immutableList).

Math static methods

Math.ceil(double) — rounds up, returns a double (Math.ceil(3.5) → 4.0). Math.floor(double) — rounds down, returns a double (Math.floor(3.5) → 3.0). Math.abs(number) — absolute value, works for int or double.

Converting between types

Method	Notes
Integer.valueOf(String)	parses to an int-backed object; throws if the string isn't a valid number
Double.valueOf(String)	parses to a double-backed object; throws if invalid
String.valueOf(Object)	converts anything to its string form; safe — never throws

Integer.valueOf/Double.valueOf only work when the input is genuinely numeric-looking; String.valueOf always succeeds regardless of input.