Java

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abstract data type

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another name for class

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aggregation

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special form of association that represents an ownership relationship between two objects; models has-a relationships; represented with an empty diamond

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boxing

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converting a primitive value to a wrapper object

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class abstraction

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separates class implementation from how the class is used

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class encapsulation

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details of class implementation are encapsulated and hidden from the user

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class's contract

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collection of methods and fields that are accessible from outside the class, together with the description of how these members are expected to behave

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composition

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when an object is exclusively owned by an aggregating object; represented with a filled in diamond

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has-a relationship

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relationship representing ownership; aggregation

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multiplicity

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may be specified by each class involved in an association; placed at the side of the class to specify how many of the class's objects are involved in the relationship in UML

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stack

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a data structure that holds data in a last-in, first-out fashion

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unboxing

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converting a wrapper object to a primitive value

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procedural paradigm

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focuses on designing methods

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object-oriented paradigm

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couples data and methods together into objects

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association

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general binary relationship that describes an activity between two classes

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aggregating object

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owner object in aggregation whose class is aggregating class

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aggregated object

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subject object in aggregation whose class is aggregated class

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wrapper classes

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way to incorporate a primitive data type into an object

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immutable

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internal values cannot be changed once an object is created

wrapper classes; BigInteger and BigDecimal classes; String objects

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radix

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2-binary; 8-octal; 16-hexadecimal

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autoboxing

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when the compiler automatically boxes a primitive value that appears in a context requiring an object

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autounboxing

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when the compiler automatically unboxes an object that appears in a context requiring a primitive value

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interned string

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a unique instance for string literals with the same character sequence that improves efficiency and saves memory

String s1 = "Welcome to Java";

String s2 = new String("Welcome to Java");

String s3 = "Welcome to Java";

s1==s2: false

s1==s3: true

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regular expression (regex)

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a string that describes a pattern for matching a set of strings

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StringBuilder vs. StringBuffer

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similar classes where one has synchronized methods (only one task is allowed to execute the methods)

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inheritance

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defining new classes from existing classes; models an is-a relationship

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subclass

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a class C1 extended from a class C2; child, extended, or derived class; not a subset-includes more methods

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superclass

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a class C2 that has class C1 extended from it; parent or base class

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private data fields

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cannot be accessed outside their class except through getters and setters

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single inheritance

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Java class may inherit directly from only one superclass

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super keyword

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refers to the superclass and can be used to invoke the superclass's methods and constructors

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method overriding

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when a subclass modifies the implementation of a method defined in the superclass; same signature/return type; reside in different classes related by inheritance

instance method only if accessible

static method cannot be-superclass method becomes hidden

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method overloading

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multiple methods with the same name but different signatures/parameters; reside in the same or different classes related by inheritance

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Object class

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where every class in Java is descended from

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polymorphism

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a variable of a supertype can refer to a subtype object

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three pillars of object-oriented programming

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encapsulation, inheritance, and polymorphism

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declared type

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the type that declares a variable

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actual type

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the actual class for the object referenced by the variable

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dynamic binding

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a method can be implemented in several classes along the inheritance chain; invoked method is determined by the variable's actual type

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casting object

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one object reference can be typecast into another object reference

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implicit casting

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Object o = new Student();

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explicit casting

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Student b = (Student)o;

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upcasting

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casting an instance of a subclass to a variable of a superclass

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downcasting

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casting an instance of a superclass to a variable of its subclass; explicit casting must be used

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equals method

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checks whether two reference variables point to the same object; tests whether two objects have the same contents

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==

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compares two primitive data type values; determines whether two objects have the same references

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type inference

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the compiler identifies the type from the variable declaration

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visibility of data and methods

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private->default->protected->public

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public modifier

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accessed from the same class; accessed from the same package; accessed from a subclass in a different package; accessed from a different package

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protected modifier

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accessed from the same class; accessed from the same package; accessed from a subclass in a different package

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default modifier

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accessed from the same class; accessed from the same package

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private modifier

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accessed from the same class

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final modifier

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prevents classes from being extended