Unit 3: Input, Strings, State, and Errors

Programs become interesting when values are not all hard-coded. Input creates a new problem: the program receives data, but receiving characters does not prove those characters represent a valid age, score, command, or measurement.

This unit connects four ideas:

• line-oriented input with Java 25 IO;
• parsing text into typed values;
• useful String operations;
• failure classification before debugging.

The unit ends with your first mastery checkpoint. You will have to predict behavior, run the program, preserve a failure, classify it, and correct the smallest responsible part.