AP Computer Science A: Using Objects and Methods

What you must know

Every AP CSA object question rests on a few ideas: a reference variable holds the address of an object, not the object itself; the dot operator calls a method or reads a field; and method calls pass arguments whose types must match the parameter list. Know the difference between a class (the blueprint) and an object (an instance). Understand that String objects are immutable, so methods like substring, toUpperCase, and replace return a new string and leave the original unchanged. Be fluent with String methods (length, indexOf, substring, equals, compareTo), Integer/Double parsing and boxing, and the Math class (Math.random, Math.abs, Math.pow).

Traps

• Using == to compare strings. == compares references; equals compares contents. new String("hi") == "hi" is usually false.
• Forgetting that substring(a, b) includes index a but excludes index b, and that an out-of-range index throws an exception.
• Assuming a method mutates a String when it actually returns a new one.
• Mixing up integer and floating-point division: 7 / 2 is 3, but 7 / 2.0 is 3.5.
• Passing an object reference to a method: the method can mutate the object's state, but reassigning the parameter does not affect the caller's variable.

Rule of thumb

When a question shows a chain of String calls, evaluate left to right, tracking the current value after each call, because each call produces a fresh string.

Worked example: String s = "Computer"; String t = s.substring(3, 6).toUpperCase(); - substring(3,6) is "put", and toUpperCase gives "PUT"; s is unchanged.