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OOD Cheat Sheet (DRAFT) by

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Design Principles

1. Javadoc all public classes and methods. Class comment should be at least two sentences, and provide inform­ation not already clear from its defini­tion.
2. Use interface types over concrete classes wherever possible. Exception: immutable "­val­ue" objects. Classes with no interface.
3. Fields must always be private. Exception: constants. Methods, classes should be as private as possible.
4. Class should never have public methods not in the interface (aside from constr­uctor).
5. Compos­ition over inheri­tance.
6. Catch and handle­/report errors as early as possible. Use Java compiler checks, enums, final first, runtime checks second.
7. Use class types over strings.
8. Check inputs.
9. Use exceptions only for except­ional situations -- not for flow control.
10. Checked vs unchecked: checked: reasonable expect­ation that the program can recover. Unchecked: programmer error (may still be recove­rable).
11. Don't leave things in an incons­istent state for any substa­ntive length of time.
12. Beware of refere­nces, copies, and mutation. Make defensive copies.
13. Separate respon­sib­ili­ties: one class, one respon­sib­ility.
14. Use class hierar­chies and dynamic dispatch over tagged classes, complex if/switch statem­ents.
15. Don't duplicate code.
16. Open for extension, closed for modifi­cation: make changes without modifying existing code; write code to support later changes without modifi­cation.
17. Extens­ibi­lity: design to make likely later changes easier.
18. Write tests first, cover the range of situat­ions, edge cases. Write code to be testable (avoid System.out); do not expose fields or add public methods just to allow for testing.
19. Loose coupling over tight coupling (avoid System.out). Write reusable components when possible.
20. You can't change an interface once it's published.
21. If you override equals(), override hashCo­de(), and vice-v­ersa.
22. Reuse existing except­ions, classes, libraries, and designs.

Scanner Methods

public boolean hasNext() Returns true if the scanner has another token in its input
public String next() Finds and returns the next complete token from the sc (throws NoSuchElement if no tokens and IllegalState if scanner is closed
Scanner(Readable source) Constructs a new Scanner that produces values scanned from the specified source.
Readable r = new StringReader(String);
Appendable a = new StringBuilder(); THROWS EXCEPTION


value is small
A logical statement is a claim that is true or false
value never decreases
The instan­tanous state of an object is the combin­ation of values of all its fields at some point in time
value is an int
The invariant is ensured by constr­uctors in the sense that whenever a public constr­uctor returns, the logical statement holds
Preserving the logical statement means that the method doesn't introduce nonsense - instead, we know that if given a object in a good state then it will leave the object in a good state as well
Enables a form of reasoning called rely-g­uar­antee.
- If the constr­uctor ensures some property
- and every method preserves the property
- then every public method, on entry, can rely on the property

Compos­ition over inheri­tance

- Composition over inheritance
- delegate design pattern: take the previous code we want to use and make it a field instead of extending it 
- has-a instead of is-a 
- always get copies of every private field instead of passing in the real thing

public boolean remove(int i) {
    return deletgate.remove(i);

public boolean contains(int i) {
   return delegate.contains(i);

Map methods

containsKey(Object key)
containsValue(Object value)
entrySet() returns a set view of the mappings (Set<Map.Entry<K, V>>)
equals(Object o)
get(Object key)
keySet() returns a set of the keys Set<K>
put(k key, V value) Associates the value with the key
putAll(Map<? extends K, ? extends V> m) Copies into new map 
remove(Object key)
values() Returns a Collection view of the values (Collection<V>)


public boolean equals(Object that) {
   if (this == that) {
      return true;
   if (!(that instanceof Duration)) {
      return false;
   return ((Duration) that).inSeconds() == this.inSeconds();

Stack Methods

empty()  Tests if this stack is empty (boolean)|
peek() Looks at top object of stack without removing it (E) |
pop() Removes the object at the top of this stack and returns that object (E)| 
push(E item) Pushes an item onto the top of this stack (E) |
search(Object o) Returns the 1-based position where an object is on this stack (int)  |
add, add, addAll, addAll, addElement, capacity, clear, clone, contains, containsAll, copyInto, elementAt, elements, ensureCapacity, equals, firstElement, get, hashCode, indexOf, indexOf, insertElementAt, isEmpty, iterator, lastElement, lastIndexOf, lastIndexOf, listIterator, listIterator, remove, remove, removeAll, removeAllElements, removeElement, removeElementAt, removeRange, retainAll, set, setElementAt, setSize, size, subList, toArray, toArray, toString, trimToSize
Deque<­E> is an interface (double ended queue)
Deque<­Int­ege­r> stack = new ArrayD­equ­e<I­nte­ger­>();


- check for overflow 
- canonicalize means converting data with multiple representations into a standard or normal form
- bug may be that a method used a non-copy of something
- interfaces can be extended to add methods to something


Static: Java's version of global variables
Static Methods: Called on the class rather than an instance and thus dont have a this to work on. Ex: Long.hashCode()
Static classes: behave like normal classes just nested in its enclosing classes namespace. Outer.nested is how you refer to it. Outer.nested can see outers private members and vise versa
- constants should be public static final and in all caps


public int hashCode() {
   return Object.hash(field, field, field);
Must use fields that equals uses


protected abstract FreecellOperations<Card> freecellModel();

  public static class SingleMove extends AbstractFreecellModelTests2 {

    protected FreecellOperations<Card> freecellModel() {
      return FreecellModelCreator.create(GameType.SINGLEMOVE);

  public static class MultiMove extends AbstractFreecellModelTests2 {
    protected FreecellOperations<Card> freecellModel() {
      return FreecellModelCreator.create(GameType.MULTIMOVE);


- an array is a mutable, fixed-length, constant-time-indexed sequence of values of type t
- new int[]{2, 4, 6, 8} gives you fixed size array
- new int[9]{} gives you a empty array of 9 spaces with null or 0
- uses length function
- mutability (intArray[3] = 17;) means int at index 3 is now 17
- assertArrayEquals