code: public void toContain(String substr) {
  		Arguments.ensureNotNull(substr, "substr cannot be null");
  		expectNotNull(this.getValue(), "Expected null to contain '%s'", substr);
  		expectTrue(this.getValue().contains(substr), "Expected '%s' to contain '%s'", this.getValue(), substr);
  	}
docs: Checks if the stored {@code String} contains a given sub string.
  <p>This method throws an {@code AssertionError} if
  <ul>
  <li>the stored {@code String} does not contain {@code substr}</li>
  <li>the stored {@code String} is {@code null}</li>
  </ul>
  @throws java.lang.IllegalArgumentException if {@code substr} is {@code null}.
  @param substr the sub string