Today, I want to write blog post to summarize all the important review notes for Oracle Certification Associate (OCA) - Java SE 8 Programmer. This post will be separated into several subjects. They’re:

1. Java Basics
2. Java Data Types
3. Methods and Encapsulation
4. Java Core APIs
5. Flow control
6. Inheritance
7. Exception Handling

Java Basics

• Check access modifier
• Check non-access modifier
• Check return type
• Check method type (constructor, getter / setter)
• Check parameter types

public static void main(String[] args) { ... }
// ^   ^      ^    ^    ^
// |   |      |    |    +- parameter type(s)
// |   |      |    +- method type
// |   |      +- return type
// |   +- non-access modifier
// +- access modifier

Java Packages

• An import statement allows to import a class or interface.
• An import statement allows to import a package using wildcard *.
• An import static statement allows to import a static method.
• An import static statement allows to import all the static methods of the target class using wildcard *.
• If a package statement is present, it must be the first non-commented line of code in the file.

Compilation

• If a variable declaration fails to compile, then all the following lines which use this variable will fail to compile, because variable is undefined.
• A local variable must be initialized before being used. Otherwise, Java fails to compile at those lines where this variable is used.
• A local variable is not required to be initialized if it is not used.

Java Data Types

Primitives

• Characters char can be concatenated, such as 'a' + 'b'. When doing so, they are considered as integer using their corresponding value in ASCII table.
• Binary number starts with the prefix 0b or 0B, e.g. 0b1101.
• Octal number starts with the prefix 0, e.g. 011.
• Decimal number does not start with 0, e.g. 11.
• Hexadecimal number starts with the prefix 0x, e.g. 0xAAA.
• A literal ends with a character could be a float, a double, a long, or a hexadecimal number.
• A long can be assigned using an integer int, because Java knows how to widen the type.
• An integer cannot be assigned as a decimal number automatically.
• Wrapper classes Byte, Short, Integer, and Long cache objects with values in the range of -128 to 127.
• Wrapper class Character caches objects with values 0 to 127.
• Wrapper class Boolean has 2 cached instances, Boolean.TRUE and Boolean.FALSE. They’re accessible directly because only two exist.
• Int is not defined in the Java API, the correct wrapper class for int is Integer.
• Bool is not defined in the Java API, the correct wrapper class for boolean is Boolean.

Operators

• The logical operator AND && has a higher operator precedence than the operator ||.

Methods and Encapsulation

Constructor

• Initializer blocks are used to initialize the variables of anonymous classes.
• A constructor must not define any return type.
• A default constructor (a public, non-argument constructor) is generated by JVM if and only if there’s no constructor defined by user.
• A constructor does not have return type, e.g. void
• If a subclass does not make an explicit call to a parent constructor, the compiler attempts to compensate for this by insert a call to the default no-argument constructor super(). However, this will fail if there’s no such constructor in the parent class.
• Subclass must make an explicit call to the parent constructor with the super() command if parent does not have a no-argument constructor.
• Overloaded constructors cannot be defined by just a change in the access levels.
• A constructor can call another overloaded constructor by using the keyword this.
• A constructor cannot invoke a constructor by using its class’s name.
• If present, the call to another constructor must be the first statement in a constructor.

public class A {
  // Method A
  public void A() { ... }

  // Constructor A
  public A() { ... }
}

Interface

• Interface methods are assumed to be public and abstract even if they’re not written as modifier. Therefore, a class implementing the methods of an interface must follow the same accessibility: public.
• Interface method cannot be marked static or default if it is already marked abstract.
• A static method in an interface can’t be called using a reference variable. It must be called using the interface name.
• Unlike an interface, if you define a static method in a base class, it can be accessed using either a reference variable or the class name.

Method

• Overloaded method means the method name is the same and the method parameter list is different. Anything else is allowed to vary. Remember that Java is case sensitive.
• Overloaded methods may or may not define a different return type.
• Overloaded methods may or may not define different access levels.
• Return type is not part of a method signature.
• If you try to execute a method using values that can be passed to multiple overloaded methods, in this case, the code will fail to compile.
• Java prefers the most specific overloaded signature it can find.
• Java prefers a single object over a vararg parameter.
• Java prefers an autoboxed parameter over a vararg parameter.
• The return statement need not be the last statement in a method, but it must be the last statement to execute in a method.
• If the compiler determines that a return statement isn’t the last statement to execute in a method, the method will fail to compile.
• If the return type of a method is int, the method can return a value of type byte.
• If a char variable gets expanded among a list of overloaded methods, it will be expanded to its nearest type, e.g. int instead of long.
• If the return type of a method is int, the method can return a value of type byte.

Garbage Collection

• You can only determine which objects are eligible to be garbage collected.
• You cannot determine when a particular object will be garbage collected. The execution of a garbage collector is controlled by the JVM.
• Removing all references to an object makes the object eligible for garbage collection, but the rules of garbage collection do not guarantee when the this deletion from memory will occur.
• The method java.lang.Object#finalize() has protected modifier.
• The method finalize is called when the object is first attempted to be garbage collected.
• The method finalize will not be called more than once for the same instance.
• A group of instances with no external references forms an island of isolation. which is eligible for garbage collection.
• A garbage collector is a low-priority thread, and its exact execution time will depend on the OS. The OS will start this thread if it needs to claim unused space. You can only be sure how many objects are eligible for GC.

Java Core APIs

String

• All the string literals are automatically instantiated into a String object.
• Whenever the JRE receives a new request to initialize a String variable using the assignment operator, it checks whether a String object with the same value already exists in the string pool.
• String objects created using the operator new are never placed in the string pool.
• The method substring does not include the character at the end position.

String Builder

• StringBuilder and StringBuffer have the same methods.
• StringBuilder has a constructor without any parameter.
• StringBuilder has a constructor with a customized capacity as int.
• StringBuilder has a constructor with a default capacity and an initial word as String.
• StringBuilder does not have method trim.
• StringBuilder does not have method concat.
• StringBuilder#subSequence(int, int) does not modify the content of builder, a new string is returned.

Arrays

• An array itself is an object.
• The creation of an array involves three steps: declaration of an array, allocation of an array, and initialization of array elements.
• An array is an object, so it’s allocated using the keyword new.
• Elements of an array that store primitive data types store 0 for integer types (byte, short, int, long).
• Elements of an array that store primitive data types store 0.0 for decimal types (float, double).
• Elements of an array that store primitive data types store false for boolean.
• Elements of an array that store primitive data types store /u0000 for char data.
• A multidimensional array can be asymmetrical. it may or may not define the same number of columns for each of its rows.
• Array anonymous initializer is only allowed in the declaration.

String[] arr = { "A", "B", "C" }; // OK
arr = { "C", "B", "A" }; // Does not compile!

ArrayList

• ArrayList#remove(Object o) removes the first occurrence of the specified element from this list, if it is present.
• Using generics only on one side in a declaration is allowed, so List<Integer> nums = new ArrayList(); compiles. However, this is not suggested. An unchecked warning will be issued by the compiler.
• It’s not possible to remove elements from an ArrayList while iterating through it using a for loop.
• indexOf(Object o) returns the index of the first occurrence of the specified element in the list, or -1 if the list doesn’t contain the element.
• An ArrayList can store any type of object.
• ArrayList#contains(Object) compares value and not reference.
• ArrayList has overridden the default toString method, so an empty array list prints [].

Date

• LocalDate instances are immutable.
• All the methods that seem to manipulate LocalDate’s value return a copy of the LocalDate instance on which it’s called.
• The withXX methods return a copy of LocalDate’s value replacing the specified day, month, or year in it.
• LocalTime stores time to nanosecond precision.
• LocalDate doesn’t define a plus() method, which accepts an integer value to be added to it. You should use plusXXX where the expression can be days, weeks, months, years.
• Cannot use a DateTimeFormatter to format a date object, because it has no time.

Lambda Expression

• Parentheses can be omitted ONLY if there’s one parameter and the parameter type is not declared. So, (String s) -> ... or s -> ....
• When braces are used around the body, the return keyword and the semicolon are required. For example, p -> { return p.getAge() < 5; }
• Lambdas work only with functional interfaces—interfaces that define exactly one abstract method.
• Each lambda expression has multiple optional and mandatory sections:
  • Parameter type (optional)
  • Parameter name (mandatory)
  • Arrow (mandatory)
  • Curly braces (optional)
  • Keyword return (optional)
  • Lambda body (mandatory)
• The return type of the functional method test in the functional interface Predicate is boolean. If you try to return another type, the code fails to compile. So Predicate<String> p = (s) -> s == null ? "A" : "B"; is incorrect.
• A simple lambda expression could be written in one line: Validate v = p -> p.getAge() <= 26;

Flow control

• The braces {} are required for methods, try blocks, and catch blocks even if there’s only one statement inside.
• If there’re duplicate constant values in a switch statements, all the duplicate cases will raise a compile error.
• The default case executes only of no matching values are found. In this case.

Inheritance

Interface

• All interface variables are implicitly assumed public static final. For these variables, they mush declare a value when they are initialized. Otherwise, the compilation fails.

Method

• The access modifier of an inherited method cannot be more restrictive than its parent.

Exception Handling

• Exceptions are divided into three categories: checked exceptions, runtime (unchecked) exceptions, and errors.
• You should not handle errors.
• If a method throws a checked exception, it must be either handled by the method or specified in its throws clause.
• If a method throws a runtime exception, it may include the exception in its throws clause.
• An exception is an object of the class java.lang.Throwable.
• When calling a method having checked exception(s), you must handle them properly, e.g. throws from method or adding a try ... catch block.
• If a method declares to throw a checked exception, its body can’t throw a more general exception.
• Error is not a checked or unchecked exception.
• If the creation of an object calls itself recursively without an exit condition, it will result a java.lang.StackOverflowError. For example: class MyClass { MyClass my = new MyClass(); }.
• ExceptionInInitializerError may be thrown by the JVM when a static initializer in your code throws a NullPointerException.

Other notes

• Consider yourself as the Java Compiler
• Read the question carefully. Be careful about the assumption, or the given context.
• Read all the options before answer a question. Sometimes multiple choices are correct.
• Be careful about the name of variable, e.g. s1 and s2 may be used in a mixed way to confuse you.
• You cannot assume the import statement if the code snippet displayed starts from line 1. (No previous lines can be hidden in this case.)
• Be careful about the application entry point, it must be public static void where the position of public static can be changed.

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