Java Primer II, Lecture notes of Java Programming

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Java Primer II

CMSC 202

Expressions

• An expression is a construct made up of

variables, operators, and method invocations,

that evaluates to a single value.

• For example:

int cadence = 0;

anArray[0] = 100;

System.out.println("Element 1 at index 0: " + anArray[0]);

int result = 1 + 2;

System.out.println(x == y? "equal" :"not equal");

Statements

• Statements are roughly equivalent to sentences

in natural languages. A statement forms a

complete unit of execution.

• Two types of statements:

– Expression statements – end with a semicolon ‘;’

• Assignment expressions
• Any use of ++ or --
• Method invocations
• Object creation expressions

– Control Flow statements

• Selection & repetition structures

Comment Types

• End of line comment – ignores everything else on the line after the

“//”

• Multi-line comment — must open with “/” and close with “/”
• Javadoc comment — special version of multi-line comment that

starts with “/**”

• Used by Java’s documentation tool

// compute the volume

/*

• sort the array using
• selection sort */

/**

• Determines if the item is empty
• @return true if empty, false otherwise */

If-Then Statement

• The if-then statement is the most basic of all

the control flow statements.

if (x == 2) System.out.println("x is 2"); System.out.println("Finished");

if x == 2: print "x is 2" print "Finished"

Python Java

Notes about Java’s if-then :

• Conditional expression must be in parentheses
• Conditional expression must result in a boolean value

Multiple Statements

• What if our then case contains multiple

statements?

if(x == 2) System.out.println("even"); System.out.println("prime"); System.out.println("Done!");

if x == 2: print "even" print "prime" print "Done!"

Python Java

Notes:

• Unlike Python, spacing plays no role in Java’s

selection/repetition structures

• The Java code is syntactically fine – no compiler errors
• However, it is logically incorrect

Blocks

• A block is a group of zero or more statements

that are grouped together by delimiters.

• In Java, blocks are denoted by opening and

closing curly braces ‘{’ and ‘}’.

if(x == 2) { System.out.println("even"); System.out.println("prime"); } System.out.println("Done!");

Note:

• It is generally considered a good practice to include the curly

braces even for single line statements.

Variable Scope

• That set of code statements in which the variable

is known to the compiler.

• Where a variable it can be referenced in your

program

• Limited to the code block in which the variable is

defined

• For example:

if(age >= 18) { boolean adult = true; } /* couldn't use adult here */

If-Then-Else Statement

• The if-then-else statement looks much like it

does in Python (aside from the parentheses

and curly braces).

if(x % 2 == 1) { System.out.println("odd"); } else { System.out.println("even"); }

if x % 2 == 1: print "odd" else: print "even"

Python Java

If-Then-Else If-Then-Else Statement

• Again, very similar…

if(x < y) { System.out.println("x < y"); } else if (x > y) { System.out.println("x > y"); } else { System.out.println("x == y"); }

if x < y: print "x < y" elif x > y: print "x > y" else: print "x == y"

Python Java

Switch Statement

• Unlike if-then and if-then-else , the switch

statement allows for any number of possible

execution paths.

• Works with byte , short , char , and int primitive

data types.

– As well as enumerations (which we’ll cover later)

Switch Statement

int cardValue = /* get value from somewhere */; switch(cardValue) { case 1: System.out.println("Ace"); break; case 11: System.out.println("Jack"); break; case 12: System.out.println("Queen"); break; case 13: System.out.println("King"); break; default: System.out.println(cardValue); }

Notes:

• break statements are typically used to terminate each case.
• It is usually a good practice to include a default case.

Switch Statement

switch (month) { case 1: case 3: case 5: case 7: case 8: case 10: case 12: System.out.println("31 days"); break; case 4: case 6: case 9: case 11: System.out.println("30 days"); break; case 2: System.out.println("28 or 29 days"); break; default: System.err.println("Invalid month!"); break; }

Note:

• Without a break statement, cases “fall through” to the next statement.

While Loops

• The while loop executes a block of statements

while a particular condition is true.

• Pretty much the same as Python…

int count = 0; while(count < 10) { System.out.println(count); count++; } System.out.println("Done!");

count = 0; while(count < 10): print count count += 1 print "Done!"

Python Java

Do-While Loops

• In addition to while loops, Java also provides a

do-while loop.

– The conditional expression is at the bottom of the

loop.

– Statements within the block are always executed

at least once.

– Note the trailing semicolon!

int count = 0; do { System.out.println(count); count++; } while(count < 10); System.out.println("Done!");

For Loop

• The for statement provides a compact way to iterate

over a range of values.

• The initialization expression initializes the loop – it is

executed once, as the loop begins.

• When the termination expression evaluates to false,

the loop terminates.

• The increment expression is invoked after each

iteration through the loop.

for (initialization; termination; increment) { /* ... statement(s) ... */ }

For Loop

• The equivalent loop written as a for loop

– Counting from start value (zero) up to (excluding)

some number (10)

for(int count = 0; count < 10; count++) { System.out.println(count); } System.out.println("Done!");

for count in range(0, 10): print count print "Done!"

Python

Java

For Loop

• Counting from 25 up to (excluding) 50 in steps

of 5

for(int count = 25; count < 50; count += 5){ System.out.println(count); } System.out.println("Done!");

for count in range(25, 50, 5): print count print "Done!"

Python

Java

For Loop

• Iterating over the contents of an array

String[] items = new String[]{"foo","bar","baz"}; for (int i = 0; i < items.length; i++) { System.out.printf("%d: %s%n", i, items[i]); }

items = ["foo", "bar", "baz"] for i in range(len(items)): print "%d: %s" % (i, items[i])

Python

Java

For Each Loop

• Java also has a second form of the for loop known

as a “for each” or “enhanced for” loop.

• This is much more like Python’s for-in loop.

• The general form is:

• For now, we’ll assume that the collection is an

array (though there are other objects it can be,

which we’ll discuss later in the semester).

for ( : ) { /* ... do something with item ... */ }

For Each Loop

• Iterating over the contents of an array using a

for-each loop

String[] items = new String[]{"foo","bar","baz"}; for(String item : items) { System.out.println(item); }

items = ["foo", "bar", "baz"] for item in items: print item

Python

Java

Reading From the Console

• Java’s Scanner object reads in input that the user

enters on the command line.

• System.in is a reference to the standard input buffer.

• We can read values from the Scanner object using the

dot notation to invoke a number of functions.

• nextInt() — returns the next integer from the buffer
• nextFloat() — returns the next float from the buffer
• nextLine() — returns the entire line as a String

Scanner input = new Scanner(System.in);

Scanner Notes

• In order to use the Scanner class, you’ll need

to add the following line to the top of your

code…

• You should never declare more than one

Scanner object on a given input stream.

• The Scanner object will wait for a user to

type, and read all text entered up until the

user presses the “enter” key (including the

newline character).

import java.util.Scanner;

Reading from the Console

• Let’s assume the user has entered “128 10”.
• The first call to nextInt() reads the characters “ 128 ” leaving “ 10\n” in the input buffer.
• The second call to nextInt() reads the “ 10 ” and leaves the “\n” in the buffer.

‘ 1 ’ ‘ 2 ’ ‘ 8 ’ ‘ ’ ‘ 1 ’ ‘ 0 ’ ‘\n’ …

System.out.print("Enter 2 numbers to sum: "); Scanner input = new Scanner(System.in); int n1 = input.nextInt(); int n2 = input.nextInt(); System.out.printf("%d + %d = %d", n1, n2, n1 + n2);

Reading via UNIX Redirection

• The Scanner class also has a bunch of hasNextX()

methods to detect if there’s another data item of the

given type in the stream.

• For example, this is useful if we were reading an

unknown quantity of integers from a file that is

redirected into our program (as above).

% cat numbers 1 2 3 4 5 6 7 8 % java Sum < numbers Sum: 36 %

int sum = 0; Scanner input = new Scanner(System.in); while(input.hasNextInt()) { sum += input.nextInt(); } System.out.println("Sum: " + sum);