Generics & Inner Classes in Java. Practical Tasks

Add the static modifier to the inner class Query and the line that prints the user’s login stops compiling: non-static variable login cannot be referenced from a static context. Differences like this between the kinds of nested classes, and between bounded type parameters, stick faster when you solve problems than when you only read the theory.

This page has 10 Java generics and inner classes exercises with hints. The first five practice nested classes: a member inner class, a local class, a static nested class and two anonymous classes. The other five practice generics: a generic class with bounded type parameters, a generic matrix, a generic class for the minimum and maximum of an array, a calculator with generic static methods, and wildcards. Five tasks have a video walkthrough (narrated in Russian), and the tasks on the matrix, minimum and maximum, calculator and wildcards include the expected output. The theory is in the lessons Inner Classes in Java: Types and Examples and Java Generics: Type Parameters, Wildcards, Erasure.

Member Inner Class: User and Query

1. Create a class User with private fields login and password and a constructor that initializes them.
2. Create a member inner class Query inside User. The method printToLog() of Query prints a message saying that the user with this login and password sent a request.
3. The class User has a method createQuery() that creates a Query object and calls printToLog().
4. In main(): create a User object and call createQuery(); create a Query object with user.new Query() and call printToLog(); create another Query object with new User(...).new Query() and call printToLog().

Example output for new User("anna", "qwerty"):

User anna with password qwerty sent a request

Video walkthrough →

Local Class

Rewrite the task Member Inner Class so that Query becomes a local class.

1. Declare the class Query in the body of the method createQuery() of User; create its object there and call printToLog().
2. Add a parameter String requestText to createQuery() and print it in printToLog() together with the login.
3. Try to assign a new value to requestText inside createQuery(). Explain why the code no longer compiles.
4. Which lines of main() from the original task stop compiling now, and why?

Video walkthrough →

Static Nested Class

Rewrite the class Query from the task Member Inner Class as a static nested class.

1. Declare Query inside User with the static modifier. The method printToLog() still prints the login and the password of the user.
2. In main(), create a Query object without user.new Query() — with new User.Query(...) — and call printToLog().
3. The method createQuery() of User must work as before.

Video walkthrough →

Anonymous Class: Printable

1. Create an interface Printable with a method void print() (or reuse one from your earlier solutions on interfaces).
2. Create an anonymous class that implements Printable. Call its print() method.
3. Replace the anonymous class with a lambda expression. Explain why this is possible for Printable but not for an interface with two abstract methods.

Video walkthrough →

Anonymous Class: Cookable

1. Create an interface Cookable with a method void cook(String str).
2. Create a class Food with the method public void prepare(Cookable c, String str) { c.cook(str); }
3. Create a Food object and call its method prepare().
4. Pass an instance of an anonymous class that implements Cookable to prepare().
5. Call prepare() once more with a different anonymous class that prints the string in capital letters. Create it right in the argument of the call, without an intermediate variable.

Generic Class with Three Type Parameters

1. Create a generic class with three type parameters (T, V, K).
2. The class has three fields of types T, V, K, a constructor that takes values of these types, and methods that return the three fields. Add a method that prints the class names of the three fields.
3. Bound the type parameters: T must implement Comparable (wrapper classes, String), V must implement Serializable and extend the class Animal, K must extend the class Number.
4. Create the class Animal and its subclass Cat that implements Serializable. In main(), create an object of your class with the type arguments String, Cat, Integer and print the class names of the fields.
5. Check that a declaration with String in place of K does not compile.

Video walkthrough →

Generic Matrix

Write a generic class Matrix<T extends Number> for a rectangular matrix of numbers. If you have already written a matrix class for int, rewrite it.

1. The elements are stored in a two-dimensional array T[][] passed to the constructor. If the rows of the array have different lengths, the constructor throws IllegalArgumentException.
2. Methods: T get(int row, int col), int rows(), int cols() and void print(), which prints each row of the matrix with elements separated by a space.
3. The method Matrix<Double> add(Matrix<? extends Number> other) adds two matrices of the same size; the element type of the second matrix may differ. For different sizes it throws IllegalArgumentException.
4. The method Matrix<Double> multiply(Matrix<? extends Number> other) multiplies two matrices. If the number of columns of the first matrix is not equal to the number of rows of the second, it throws IllegalArgumentException.
5. Test the class on these matrices: a is Integer [[1, 2], [3, 4]], b is Integer [[5, 6], [7, 8]], c is Double [[0.5, 0.5], [0.5, 0.5]].

The output of a.add(c).print() and a.multiply(b).print():

1.5 2.5
3.5 4.5
19.0 22.0
43.0 50.0

Minimum and Maximum of an Array

Write a generic class MinMax with methods that find the minimum and the maximum element of an array. The array is a field of the class and is passed to it through the constructor.

1. Bound the type parameter so that the class works with any comparable elements: Integer, Double, String.
2. The methods T min() and T max() return an element of the array. Compare elements with compareTo(), not by converting them to double.
3. For an empty array the methods throw NoSuchElementException with a clear message, and a null array causes NullPointerException in the constructor (Objects.requireNonNull()).
4. Test the class on the arrays Integer[] {3, -7, 12, 0} and String[] {"pear", "apple", "plum"}.

Expected output:

min = -7, max = 12
min = apple, max = plum

Calculator with Generic Static Methods

Write a non-generic class Calculator with generic static methods sum(), multiply(), divide() and subtract(). The parameters of each method are two numbers, possibly of different types (for example, Integer and Double), and the method performs the operation on them. The result is a double.

1. Give each method its own type parameters bounded by Number.
2. Test the methods with the calls from the example below.
3. Check that the call Calculator.sum("1", 2) does not compile.
4. In a comment to the class, explain how your signature differs from double sum(Number a, Number b) and when a type parameter in a method signature is really needed.

System.out.println(Calculator.sum(5, 2.5));         // 7.5
System.out.println(Calculator.multiply(3, 4L));     // 12.0
System.out.println(Calculator.subtract(10, 0.25f)); // 9.75
System.out.println(Calculator.divide(7, 2));        // 3.5
System.out.println(Calculator.divide(1, 0));        // Infinity

Wildcard: Copy a List and Find the Maximum

Write a utility class ListUtils with two generic static methods and find out which calls the compiler accepts.

1. static <T> void copy(List<? super T> dest, List<? extends T> src) appends all elements of src to the end of dest.
2. static <T extends Comparable<? super T>> T max(List<? extends T> list) returns the largest element; for an empty list it throws NoSuchElementException.
3. Copy a List<Integer> [1, 2, 3] into an empty List<Number> and into a List<Object> that already contains the string "x". Print both lists. Then find the max of Arrays.asList(3, 9, 4) and Arrays.asList("pear", "apple", "plum").
4. Try to copy a List<Number> into a List<Integer>. Explain why the compiler rejects this call and which error it protects you from.
5. Write a method static void addZero(List<? extends Number> list) that calls list.add(0). Explain the compilation error and fix the signature so that the method accepts List<Integer>, List<Number> and List<Object>.

Expected output of step 3:

[1, 2, 3]
[x, 1, 2, 3]
9
plum

What Each Exercise Practices

Every task checks one rule from the module lessons: how an object of a nested class is created and what it can access, or how a bound on a type parameter restricts the arguments and methods. If a task does not work out, open the lesson section from the last column — the rule is explained there with an example.

Where Solutions Usually Go Wrong

A solution often compiles and even works on one example, yet breaks the rule the task is about: it uses a raw type, compares numbers through double, or never checks edge cases. Go through this list before you call a task done.

• new Query() in a static main(). An object of an inner class needs an object of the outer class: user.new Query(). If the class does not need the outer object, make it a static nested class — see Where Developers Get Tripped Up in the lesson.
• Raw types. Comparable without a type argument or Matrix without angle brackets switches off type checking. Compile with -Xlint:unchecked: a correct solution produces no warnings.
• Comparing with doubleValue() instead of compareTo(). Such a MinMax does not work with strings and compares large long values wrongly: values above 2⁵³ lose precision when converted to double.
• Testing only convenient data. An empty array, matrices of different sizes and division by zero are exactly the cases where the task texts require specific behavior.