Wrapper Classes in Java. Practical Tasks

The line int count = stock.get("banana"); has no explicit intValue() call, yet it throws a NullPointerException when the key "banana" is missing from a Map<String, Integer>. Bugs like this are easier to spot after a few exercises than after rereading the rules.

This page collects Java wrapper classes exercises (Integer, Long, Double, Boolean) and autoboxing practice: the valueOf(), parseXxx() and xxxValue() methods, the Integer cache and comparison with == and equals(), unboxing null, autoboxing in loops, and overload resolution with boxing and varargs. Read the module lessons first: Wrapper Classes in Java, Autoboxing and Unboxing in Java and Method Overloading with Widening, Autoboxing and Varargs.

Double Methods: valueOf(), parseDouble() and xxxValue()

Write a program that works with the Double class and prints the result of each step to the console.

1. Create Double objects using both forms of valueOf(): from a double primitive and from a string.
2. Convert the string "300.99" to a double primitive with Double.parseDouble().
3. Convert a Double object with the value 300.99 to every numeric primitive type using byteValue(), shortValue(), intValue(), longValue(), floatValue() and doubleValue(). Explain why byteValue() does not return 300.
4. Convert a literal of type double to a string: String d = Double.toString(3.14);
5. Try to parse the strings "3,14" and " 3.14 " with Double.parseDouble(). Catch the parsing error and print a message like "Not a number: ..." instead of letting the program crash.

Wrapper Class Methods Table

Fill in the table: put an "x" where the row's method exists in the column's wrapper class. In the last column, mark whether the method is static or an instance method.

Comparing Integer and Long: == or equals()

Write a program that compares wrapper objects in different ways. Before running it, write down the expected result of each line, then compare it with the actual output.

1. Declare Integer a = 127, b = 127; and Integer c = 128, d = 128;. Print the results of a == b, c == d, c.equals(d) and c <= d && c >= d.
2. Declare Long f = 128L; and print f == 128, f.equals(128) and f.equals(128L).
3. Write a static method boolean sameValue(Integer x, Integer y) that compares the values and throws no exception when one or both arguments are null (two null values are considered equal). Test it on the pairs (1000, 1000), (null, 5) and (null, null).
4. Run the program with the JVM option -XX:AutoBoxCacheMax=1000. Which output line changed, and why did the others stay the same?

Long or long: Autoboxing in a Loop

See in practice what autoboxing costs in a “hot” loop.

1. Write a method static Long sumBoxed(int n) that sums the numbers from 0 to n - 1 into a variable of type Long, and a method static long sumPrimitive(int n) with an accumulator of type long.
2. Call both methods with n = 10_000_000 and time each call with System.nanoTime(). Print the sum and the time in milliseconds. Both versions must print the sum 49999995000000.
3. Repeat the measurement 5 times in a loop and compare the time of the first and the last repetition.
4. Count how many new Long objects a call to sumBoxed(10_000_000) creates, given that Long.valueOf() caches values from -128 to 127. Write the answer as a comment in the code.

Which Overloaded Method Is Called: Widening, Boxing, Varargs

Given a class with four overloaded print() methods:

public class OverloadQuiz {
    static void print(long x)    { System.out.print("long "); }
    static void print(Integer x) { System.out.print("Integer "); }
    static void print(Object x)  { System.out.print("Object "); }
    static void print(int... x)  { System.out.print("int... "); }

    public static void main(String[] args) {
        byte b = 1;
        char ch = 'a';
        Integer boxed = 3;
        Character c = 'a';

        print(b);
        print(5);
        print(boxed);
        print(ch);
        print(c);
        print(3.0);
        print(1, 2);
        print();
        // print(null);
    }
}

1. Without running the program, write down what each call prints and which resolution phase (1, 2 or 3) selects the method. Then run the program and check your answers.
2. Add the method static void print(double x). Predict which words in the output change, and verify.
3. Uncomment the print(null) call. Will the program compile? Explain the compiler message.

What Each Exercise Practices

The first two exercises test the wrapper class methods, the next three cover autoboxing and unboxing, and the last one covers overload resolution. If an exercise does not work out, go back to the lesson section in the table: it has the rule and an example with output.

Common Mistakes in Solutions

Most mistakes in these exercises come not from unfamiliar syntax but from testing on “convenient” data: small numbers, filled-in keys, a single run. Before you call a solution done, check it on boundary values.

• Comparing two Integer objects with ==. The test passes for values from -128 to 127 and fails on real data.
• try-catch instead of a fix. Catching NullPointerException hides the bug; it is better to substitute a default before unboxing (getOrDefault(), Boolean.TRUE.equals()).
• A single timing measurement. The first run of a loop includes JIT compilation, so compare results after several repetitions.
• Choosing a method by “the closest type”. The compiler stops at the first phase where a suitable method is found, even if a later phase has a method with a closer type.