List<String> listStr = new ArrayList<>();
 listStr.contains(34); //warning

Given object can not contain instances of int (expected String)

List<?> listStr = new ArrayList<>();

public class FakeString {
  private final String value;

  public FakeString(String value) {
    if (value == null) {
      throw new IllegalArgumentException();
    }
    this.value = value;
  }

  public int hashCode() {
    return value.hashCode();
  }

  public boolean equals(Object o) {
    return value.equals(o);
  }
}

List<String> strings = Arrays.asList("foo", "bar", "baz");
System.out.println(strings.contains(new FakeString("bar")));