Cheat Sheet

This cheat sheet contains the most important takeaways that lead up to section ten.

Definitions

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• Class: blueprint from which you can create objects.

• Object: an object is a thing that you can see or describe.

• Field: class variable that describes an object.

• Action: method (function) that represents what the object can do.

• Constructor: runs when you create a new object.

  • Purpose: update every field of the newly created object.

• this: refers to the current object that's calling the constructor or method.

  • Purpose: useful when fields and parameters have conflicting names.

• Copy Constructor: runs when you create an object.

  • Purpose: copy every value from a source object into a new object.

• Getter: method (function) that returns a copy of a private field's value.

• Setter: method (function) that updates the value of a private field.

• toString: method that returns a String representation of every field in an object.

  • Purpose: When you print an object, Java internally calls the toString method.

public vs. private

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• public: provides public access to a field or method from anywhere.

  • Constructors and methods tend to be public.

• private: prevents direct access of a field or method outside of its class.

  • To protect the state of an object, fields tend to be private.

The Big Three

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If a class has fields, you need to add:

1. Constructor -- to update the fields of a new object.

2. Getters -- to get the value of a private field.

3. Setters -- to set the value of a private field.

Debugging

1. Continue: continues to the next breakpoint.
2. Step over: steps over a line (see Debugging Exercise 3).
3. Step into: steps into a function/cons.
4. Step out: steps out of a function/cons.
5. Restart: restarts the runtime.
6. Stop: stops the runtime

Exceptions

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Checked:

• Compile-time.
• Outside the application's control.
• Java forces you to try-catch the potential failure.

Unchecked:

• Runtime.
• Results from badly written code.
• Do not catch unchecked exceptions. Instead, fix your code.

Throwing Exceptions:

• Forces the caller to improve their code.
• Throw an IllegalArgumentException if the caller passes illegal values into a method/constructor.
• Throw an IllegalStateException if the caller invokes a method at a bad time (object not in a valid state).

Mutable / Immutable Objects

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• Mutable class: class with accessible mutators (setters).
• Immutable class: class without accessible mutators (setters).

Example 1:

An array is a mutable object.

Every array is a mutable object of a type[] class.

It follows that arrays are vulnerable to the reference trap.

Example 2:

String is an immutable class.

It follows that objects of the String class are immune to the reference trap.

Reference Trap

The state of a variable should not change because you updated another.

Falling into a reference trap can:

1. lead to unpredictable behavior.

2. lead to variables changing when you don't expect them to.

3. leave you wondering: why is my code doing this?

Wrapper class

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Wrapper: Immutable class that wraps around a primitive.

String is not a wrapper class. But, it is immutable.

Rule:

• Use primitive when possible (faster and less memory).
• Use wrapper only when you need to (inside ArrayList...).

Array vs. ArrayList vs. HashMap

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• Array

  • fixed.
  • stores primitives and objects.
  • type[] values = new type[]

• ArrayList

  • resizable.
  • stores objects.
  • ArrayList<object> values = new ArrayList<object>();

• HashMap

  • resizable.
  • stores key-value pairs.
  • HashMap<Object, Object> pairs = new HashMap<Object, Object>()

Rule: use arrays when size is fixed (less overhead). Use ArrayList when size can vary. Use HashMap when there is parity between data.

HashMap vs. TreeMap vs. LinkedHashMap

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Collection types that implement the Map interface.

• HashMap
  • Unordered entries.
• TreeMap
  • Entries ordered based on what you specify.
• LinkedHashMap
  • Entries ordered based on insertion.

Rule: favor HashMap because it offers the best performance. Use TreeMap or LinkedHashMap in the unique event that entries must be sorted.

Test-Driven Development.

1. Identify meaningful test cases.
2. Write a unit test for each test case.
   • Write code to make the test fail.
   • Write code to make the test pass.
   • Refactor if necessary.

@Test annotates a unit test. @Before annotates a method that runs before each test.

Stream Pipeline

Instead of loops, you should pass elements into a pipeline:

collection.stream()
	.intermediateOperation(lambda expression)
        .intermediateOperation(lambda expression)
	.intermediateOperation(lambda expression)
	.intermediateOperation(lambda expression)
        .terminalOperation(lambda expression)

Intermediate operation: processes the sequence of elements and continues the pipeline.
Terminal operation: ends the pipeline and may return a final value.
Lambda expression syntax:

((parameter) -> {
     code goes here
})

equals() method

public boolean equals(Object obj) {

    return false if parameter is null.
    
    return false if parameter isn't instance of class.

    typecast the object.
    
    compare fields from both objects and return the result.
}

hashCode() method

When you add an equals() method, you must always add a hashCode() method.

public int hashCode() {
     return Objects.hash(fields go here);    
}

Naming Conventions

• class: CamelCase
• variable: lowerCamelCase
• method: lowerCamelCase
• constant: UPPER_SNAKE_CASE

static and final

• static: variable or method belongs to the class.
• final: cannot be updated.

Inheritance

• Classes that share common fields should inherit from a parent class.
• Use super() to call the parent constructor from the child class.
• Use super.method to call a parent method from the child class.
• Interface: contract of behaviour. Classes that implement an interface must override every method inside of it.
• Polymorphism:
  • an object can take its form or the class it inherits from.
  • an object can take its form or the interface it implements.
• abstract class: Parent class that provides inheritance. The caller is forbidden from creating objects of an abstract class.
• abstract method: a child must override every abstract method.

Enums

Use an enum to ensure a variable can only be a limited number of values.

Tips and Tricks.

• Conditional assignment syntax: variable = (comparison) ? value true : value false.

• Syntax to directly pass an array into a method: new String[] { element, element }

• Syntax to directly return an array: return new String[] { element, element }

• Use the Java Code Generators extension to autogenerate getters, setters, equals(), hashCode(), and toString().