• pre-conditions (in private methods only) - the requirements which a method requires its caller to fulfill
• post-conditions - verify the promises made by a method to its caller
• class invariants - validate object state
• unreachable-at-runtime code - parts of your program which you expect to be unreachable, but which cannot be verified as such at compile-time (often else clauses and default cases in switch statements)

import java.util.Random;

public final class Flower {

  public static void main(String... arguments) {
    final Flower tulip = new Flower("Tulip", 1);
    tulip.grow();
    tulip.grow();
    System.out.println(tulip);

    tulip.randomGrowOrWither();
    System.out.println(tulip);

    tulip.wither();
    tulip.wither();
    System.out.println(tulip);
  }

  /**
  * @param aSpecies must have content.
  * @param aInitialLength must be greater than 0.
  */
  public Flower(final String aSpecies, final int aInitialLength) {
    //assert is NOT used to validate params of public methods
    if (!isValidSpecies(aSpecies)) {
      throw new IllegalArgumentException("Species must have content.");
    }
    if (!isValidLength(aInitialLength)) {
      throw new IllegalArgumentException("Initial length must be positive.");
    }

    fSpecies = aSpecies;
    fLength = aInitialLength;

    //check the class invariant
    assert hasValidState(): "Construction failed - not valid state.";
  }

  public boolean isMature() {
    return fLength > 5 ;
    //not necessary to assert valid state here, since
    //the state has not changed.
  }

  /**
  * Increase the length by at least one unit.
  */
  public void grow(){
    //this style of checking post-conditions is NOT recommended,
    //since the copy of fLength is always made, even when
    //assertions are disabled.
    //See <tt>wither</tt> (below) for an example with an improved style.
    final int oldLength = fLength;
    fLength += getLengthIncrease(fLength);
    //post-condition: length has increased
    assert fLength > oldLength;

    //check the class invariant
    assert hasValidState(): this;
  }

  /**
  * Decrease the length by one unit, but only if the resulting length
  * will still be greater than 0.
  */
  public void wither(){

    //this local class exists only to take a snapshot of the current state.
    //although bulky, this style allows post-conditions of arbitrary complexity.
    class OriginalState {
      OriginalState() {
        fOriginalLength = fLength;
      }
      int getLength() {
        return fOriginalLength;
      }
      private final int fOriginalLength;
    }
    OriginalState originalState = null;
    //construct object inside an assertion, in order to ensure that
    //no construction takes place when assertions are disabled.
    //this assert is rather unusual in that it will always succeed, and in that
    //it has side-effects - it creates an object and sets a reference
    assert (originalState = new OriginalState()) != null;

    if (fLength > 1) {
      --fLength;
    }

    //post-condition: length has decreased by one or has remained the same
    assert fLength <= originalState.getLength();

    //check the class invariant
    assert hasValidState(): this;
  }

  /**
  * Randomly select one of three actions
  * <ul>
  * <li>do nothing
  * <li>grow
  * <li>wither
  * </ul>
  */
  public void randomGrowOrWither() {
    //(magic numbers are used here instead of symbolic constants
    //to slightly clarify the example)
    Random generator = new Random();
    int action = generator.nextInt(3);
    //according to the documentation for the Random class, action
    //should take one of the values 0,1,2.
    if (action == 0) {
      //do nothing
    }
    else if (action == 1) {
      grow();
    }
    else if (action == 2) {
      wither();
    }
    else {
      //this is still executed if assertions are disabled
      throw new AssertionError("Unexpected value for action: " + action);
    }
    //check the class invariant
    assert hasValidState(): this;
  }

  /** Use for debugging only.  */
  public String toString(){
    final StringBuilder result = new StringBuilder();
    result.append(this.getClass().getName());
    result.append(": Species=");
    result.append(fSpecies);
    result.append(" Length=");
    result.append(fLength);
    return result.toString();
  }

  // PRIVATE 
  private final String fSpecies;
  private int fLength;

  /**
  * Implements the class invariant.
  *
  * Perform all checks on the state of the object.
  * One may assert that this method returns true at the end
  * of every public method.
  */
  private boolean hasValidState(){
    return isValidSpecies(fSpecies) && isValidLength(fLength);
  }

  /** Species must have content.  */
  private boolean isValidSpecies(final String aSpecies) {
    return aSpecies != null && aSpecies.trim().length()>0;
  }

  /** Length must be greater than 0.  */
  private boolean isValidLength(final int aLength) {
    return aLength > 0;
  }

  /** Length increase depends on current length. */
  private int getLengthIncrease(int aOriginalLength) {
    //since this is a private method, an assertion
    //may be used to validate the argument
    assert aOriginalLength > 0: this;
    int result = 0;
    if (aOriginalLength > 10) {
      result = 2;
    }
    else {
      result = 1;
    }
    assert result > 0 : result;
    return result;
  }
}