Bits and Pieces Some random things in Java

Bits and Pieces Some random things in Java 17 -Dec-21

Versions of Java Oak: Designed for embedded devices Java: Original, not very good version (but it had applets) Java 1. 1: Adds inner classes and a completely new event-handling model Java 1. 2: Includes “Swing” but no new syntax Java 1. 3: Additional methods and packages, but no new syntax Java 1. 4: More additions and the assert statement Java 1. 5: Generics, enums, new for loop, and other new syntax Java 1 Java 2 Java 5. 0 2

Using the command line n Although we use Eclipse, everything can be done from the command line n To compile all Java files in this directory: javac *. java n n n To run a program, where "My. Program" is a Java file containing a main method, do: java My. Program n n javac is the Java compiler; it compiles. java files to. class files are in a special, machine-independent language called Bytecode The java command invokes the Java Virtual Machine (JVM), which executes the Bytecode The above commands work if you have Java installed completely correctly, which usually means setting your CLASSPATH environment variable

Array. Lists and arrays n A Array. List is like an array of Objects, but. . . n n Arrays use [ ] syntax; Array. Lists use object syntax An Array. List expands as you add things to it Arrays can hold primitives or objects, but Array. Lists can only hold objects To create an Array. List: n Array. List<type> my. List = new Array. List<type>(); n type must be an Object type, not a primitive type n n type is a "type parameter"—it's like a parameter, but it's a type, not a value To use an Array. List, n add(value) n old. Value = set(index, new. Value) n value = get(index)

Array. Lists, then and now n Starting in Java 5, Array. Lists have been genericized n That means, every place you used to say Array. List, you now have to say what kind of objects it holds; like this: Array. List<String> n n n Other languages call generics "type parameters" If you don’t do this, you will get a warning message, but your program will still run An older type of collection, the Vector, is essentially the same as Array. List

Type wildcards n Here’s a simple (no generics) method to print out any list: n n The above still works in Java 1. 5, but now it generates warning messages n n private void print. List(List list) { for (Iterator i = list. iterator(); i. has. Next(); ) { System. out. println(i. next()); } } Java 1. 5 incorporates lint (like C lint) to look for possible problems You should eliminate all errors and warnings in your final code, so you need to tell Java that any type is acceptable: n private void print. List. Of. Strings(List<? > list) { for (Iterator<? > i = list. iterator(); i. has. Next(); ) { System. out. println(i. next()); } } 6

Auto boxing and unboxing n Java won’t let you use a primitive value where an object is required--you need a “wrapper” n n n Similarly, you can’t use an object where a primitive is required-you need to “unwrap” it n n int n = ((Integer)my. Array. List. get(2)). int. Value(); Java 1. 5 makes this automatic: n n Array. List<Integer> my. List = new Array. List<Integer>(); my. List. add(new Integer(5)); my. Array. List<Integer> my. List = new my. Array. List<Integer>(); my. List. add(5); int n = my. List. get(2); Other extensions make this as transparent as possible n n For example, control statements that previously required a boolean (if, while, do-while) can now take a Boolean There are some subtle issues with equality tests, though

Writing your own generic types n public class Box<T> { private List<T> contents; public Box() { contents = new Array. List<T>(); } public void add(T thing) { contents. add(thing); } public T grab() { if (contents. size() > 0) return contents. remove(0); else return null; n n } Sun’s recommendation is to use single capital letters (such as T) for types If you have more than a couple generic types, though, you should use better names

New for statement n n n The syntax of the new statement is for(type var : array) {. . . } or for(type var : collection) {. . . } Example: for(float x : my. Real. Array) { my. Real. Sum += x; } For a collection class that has an Iterator, instead of for (Iterator iter = c. iterator(); iter. has. Next(); ) ((Timer. Task) iter. next()). cancel(); you can now say for (Timer. Task task : c) task. cancel();

New for statement with arrays n n n The new for statement can also be used with arrays Instead of for (int i = 0; i < array. length; i++) { System. out. println(array[i]); } you can say (assuming array is an int array): for (int value : array) { System. out. println(value); } Disadvantage: You don’t know the index of any of your values

New methods in java. util. Arrays n Java now has convenient methods for printing arrays: n n n Java now has convenient methods for comparing arrays: n n n Arrays. to. String(my. Array) for 1 -dimensional arrays Arrays. deep. To. String(my. Array) for multidimensional arrays Arrays. equals(my. Array, my. Other. Array) for 1 -dimensional arrays Arrays. deep. Equals(my. Array, my. Other. Array) for multidimensional arrays It is important to note that these methods do not override the public String to. String() and public boolean equals(Object) instance methods inherited from Object n The new methods are static methods of the java. util. Arrays class 11

Enumerations n n An enumeration, or “enum, ” is simply a set of constants to represent various values Here’s the old way of doing it n n n public final int int SPRING = 0; SUMMER = 1; FALL = 2; WINTER = 3; This is a nuisance, and is error prone as well Here’s the new way of doing it: n enum Season { WINTER, SPRING, SUMMER, FALL } 12

enums are classes n An enum is actually a new type of class n n You can declare them as inner classes or outer classes You can declare variables of an enum type and get type safety and compile time checking n n enums extend java. lang. Enum and implement java. lang. Comparable n n n Each declared value is an instance of the enum class Enums are implicitly public, static, and final You can compare enums with either equals or == Hence, enums can be sorted Enums override to. String() and provide value. Of() Example: n n n Season season = Season. WINTER; System. out. println(season ); // prints WINTER season = Season. value. Of("SPRING"); // sets season to Season. SPRING 13

Advantages of the new enum n Enums provide compile-time type safety n n Enums provide a proper name space for the enumerated type n n n If you add, remove, or reorder constants, you must recompile, and then everything is OK again Enum printed values are informative n n With int enums you have to prefix the constants (for example, season. WINTER or S_WINTER) to get anything like a name space. Enums are robust n n int enums don't provide any type safety at all: season = 43; If you print an int enum you just see a number Because enums are objects, you can put them in collections Because enums are classes, you can add fields and methods 14

Enums really are classes public enum Coin { // enums can have instance variables private final int value; // An enum can have a constructor, but it isn’t public Coin(int value) { this. value = value; } // Each enum value you list really calls a constructor PENNY(1), NICKEL(5), DIME(10), QUARTER(25); // And, of course, classes can have methods public int value() { return value; } } 15

Other features of enums n values() returns an array of enum values n n switch statements can now work with enums n n Season[] season. Values = Season. values(); switch (this. Season) { case SUMMER: . . . ; default: . . . } You must say case SUMMER: , not case Season. SUMMER: It’s still a very good idea to include a default case It is possible to define value-specific class bodies, so that each value has its own methods n The syntax for this is weird, and I don’t yet understand it well enough myself to lecture on it 16

varargs n You can create methods and constructors that take a variable number of arguments n n n public void foo(int count, String. . . cards) { body } The “. . . ” means zero or more arguments (here, zero or more Strings) Call with foo(13, "ace", "deuce", "trey"); Only the last argument can be a vararg To iterate over the variable arguments, use the new for loop: for (String card : cards) { loop body } 17

Static import facility n import static org. iso. Physics. *; class Guacamole { public static void main(String[] args) { double molecules = AVOGADROS_NUMBER * moles; . . . } } n n n You no longer have to say Physics. AVOGADROS_NUMBER Are you tired of typing System. out. println(something); ? Do this instead: n import static java. lang. System. out; n out. println(something); 18

static n Instance variables belong to instances—objects—belonging to the given class n n Instance methods are executed by those instances n n Within an instance method, the word this refers to the object that is executing the method Static variables belong to the class as a whole n n n Each object has its own copy of these variables Static variables may be accessed by objects in that class, but… There is only one copy of each static variable, shared by all the objects Static methods belong to the class as a whole n n Static methods may be used by objects in that class, but… Static method cannot directly use instance variables (whose? ), instance methods (who is executing it? ), or the word this (to whom would it refer? )

Why would you ever use static? n n Consider the method int gcd(int, int), which takes two integers and returns their Greatest Common Divisor This is a handy method to have when you are working with fractions, but why should you ask some particular fraction to do the work for you? n n It makes more sense to ask the class to do the work: n n int g = three. Quarters. gcd(x, y); int g = Fraction. gcd(x, y); The gcd method does not refer to any particular fraction, or any instance variables, or the word this – so it should be static

java. util. Scanner n Java finally has a fairly simple way to read input n Scanner sc = Scanner. create(System. in); n n n n n boolean b = sc. next. Boolean(); byte by = sc. next. Byte(); short sh = sc. next. Short(); int i = sc. next. Int(); long l = sc. next. Long(); float f = sc. next. Float(); double d = sc. next. Double(); String s = sc. next. Line(); By default, whitespace acts as a delimiter, but you can define other delimiters with regular expressions 21

java. util. Formatter n n n Java now has a way to produce formatted output, based on the C printf statement String line; int i = 1; while ((line = reader. read. Line()) != null) { System. out. printf("Line %d: %s%n", i++, line); } Example specifications: n n n %8 d – decimal integer, 8 character positions %8. 4 f – floating point, 8 character positions, 4 after the decimal point %-20 s – string, 20 character positions, left justified %6 b -- boolean %n -- newline There about 45 different format specifiers (such as %d and %s), most of them for dates and times 22

Annotations n In general code: n Indicate that the method is supposed to override an inherited method @Override public boolean equals(Item other) {. . . } n n Indicate that the method should not be used in new code, because it has been replaced by something better @Deprecated public int ancient. Method() { … } n n Gives a syntax error because the signature is wrong Eclipse will give you a warning (not an error) if you try to use this method Indicate that you know about the problem but are doing something anyway @Suppresswarnings(type) // Questionable code… You can create other kinds of annotations Various uses in JUnit 4 (@Before, @Test, and several others) 23

The End