## Why is this an issue?

When the call to a function doesn’t have any side effects, what is the point of making the call if the results are ignored? In such case, either the function call is useless and should be dropped or the source code doesn’t behave as expected.

To prevent generating any false-positives, this rule triggers an issue only on the following predefined list of immutable classes in the Java API :

- `java.lang.String`
- `java.lang.Boolean`
- `java.lang.Integer`
- `java.lang.Double`
- `java.lang.Float`
- `java.lang.Byte`
- `java.lang.Character`
- `java.lang.Short`
- `java.lang.StackTraceElement`
- `java.time.DayOfWeek`
- `java.time.Duration`
- `java.time.Instant`
- `java.time.LocalDate`
- `java.time.LocalDateTime`
- `java.time.LocalTime`
- `java.time.Month`
- `java.time.MonthDay`
- `java.time.OffsetDateTime`
- `java.time.OffsetTime`
- `java.time.Period`
- `java.time.Year`
- `java.time.YearMonth`
- `java.time.ZonedDateTime`
- `java.math.BigInteger`
- `java.math.BigDecimal`
- `java.util.Optional`

As well as methods of the following classes:

- `java.util.Collection`:
  - `size()`
  - `isEmpty()`
  - `contains(...)`
  - `containsAll(...)`
  - `iterator()`
  - `toArray()`
- `java.util.Map`:
  - `size()`
  - `isEmpty()`
  - `containsKey(...)`
  - `containsValue(...)`
  - `get(...)`
  - `getOrDefault(...)`
  - `keySet()`
  - `entrySet()`
  - `values()`
- `java.util.stream.Stream`
  - `toArray`
  - `reduce`
  - `collect`
  - `min`
  - `max`
  - `count`
  - `anyMatch`
  - `allMatch`
  - `noneMatch`
  - `findFirst`
  - `findAny`
  - `toList`

### Noncompliant code example

    public void handle(String command){
      command.toLowerCase(); // Noncompliant; result of method thrown away
      ...
    }

### Compliant solution

    public void handle(String command){
      String formattedCommand = command.toLowerCase();
      ...
    }

### Exceptions

This rule will not raise an issue when both these conditions are met:

- The method call is in a `try` block with an associated `catch` clause.
- The method name starts with "parse", "format", "decode" or "valueOf" or the method is `String.getBytes(Charset)`.

    private boolean textIsInteger(String textToCheck) {

        try {
            Integer.parseInt(textToCheck, 10); // OK
            return true;
        } catch (NumberFormatException ignored) {
            return false;
        }
    }

## Resources

- CERT, EXP00-J. - Do not ignore values returned by methods