Review Session
Ryan Ly (slides by Jai Madhok)
Email: rly1@jhu.edu
Key Concepts
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Identifiers- What are they?
Rules to determine invalid vs. valid identifiers
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Cannot be a reserved word
No spaces in between
No symbols other than the ‘$’ and ‘_’
Cannot begin with a digit
Data Types
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3 kinds of primitive data types
Type conversion between data types
 Implicit vs. explicit type conversion
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The ‘Cast’ operator:
Syntax: (dataType) expression
What happens when an operator has mixed
operands?
Strings
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A String is an ‘Object’
In Java, they are enclosed in double quotes
What is a null string? Empty string?
How does indexing work?
Predefined methods:
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substring(startIndex, stopIndex)
charAt(index)
indexOf(ch)
length()
replace()
toLowerCase(), toUpperCase()
Named Constants
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Use the keyword final
Identifier should be in all Capital Letters
Memory location whose content is not allowed to
change during program execution
Ex: final int CENTS_PER_DOLLAR= 100;
Several advantages:
 Prevent typos
▪ If you mistype the name of the constant, the compiler will warn you
as compared to typing in a wrong value which the compiler will
blindly accept
 Easy to modify the program in the future
 Good programming practice
Input Statements
To put data into variables from an input device, we
first create an input stream object and associate it
with a standard input device
Scanner scan= new Scanner(System.in);
 More about Scanner:
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next()
nextInt()
nextDouble()
nextLine()
Know how to use these!
How do we read in a single character using a
Scanner object?
Tokenizing & Parsing
Sometimes we read in input in the form of a string
and then want to extract numeric data from it
 How do we do this? Tokenize and Parse
 Syntax:
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StringTokenizer t= new StringTokenizer(str, delim)
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Say we want an integer:
int a= Integer.parseInt(t.nextToken())
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Other method: Double.parseDouble()
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Note: Parsing and type casting are different things
File Handling
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A file is defined as an area in secondary storage
used to hold information
Scanner scan= new Scanner(new FileReader(filename1));
PrintWriter oFile= new PrintWriter(filename2);
Warning: Closing the file you are writing to is more
important than you think it is
 Use same methods for reading and
writing/formatting files as you have been for user
input/output
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Formatting Output With printf
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Syntax:
System.out.printf(formatString, argumentList)
 formatString: string specifying the format
 argumentList: list of arguments containing constant
values, variables, or expressions, separated by comma
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Example:
System.out.printf(“There are %.2f inches in %d centimeters
\n”, cm/2.54, cm)
 Where cm is a variable of the type int.
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Note: You can also use the DecimalFormat class to
format your decimal output. Take your pick!
More Fun With Strings
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compareTo()
 Compare Strings character by character until a mismatch
is found
 What does this method return?
 What is the basis for deciding what a mismatch is and
what it isn’t?
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equals()
 Case sensitive method
 How to solve the case sensitivity issue?
 What does the method return?
Operators- Order of Precedence
() . []
Unary Operators: negation, casting, not (!), ++, -Arithmetic Operators I: * / %
Arithmetic Operators II: + Comparison Operators: < <= > >=
Equivalence Operators: == !=
Logical AND: &&
Logical OR: ||
Ternary Operator: ?:
Assignment Operators: = += -= *= /= %=
Control Structures
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Provide alternatives to sequential program
execution and are used to alter flow of
execution
Alternatives are:
 Selection – if, if/else, switch/case
 Repetition – for, while, do-while
While Loops
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Using loop control variables
 Counter controlled while loops
 Sentinel controlled while loops
 Flag controlled while loops
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EOF controlled while loops
Do-while loops are different
from a generic while
 Run body then check condition
to decide whether to loop
T
F
for (initial statement; loop condition; update statement)
Initial
Statement
Loop
Condition
T
Statements
F
KNOW THIS LOOP COLD!
Update
Statement
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Exit early from a loop
Skip the remaining cases in a switch case
construct
Don’t try and use them at random places to
make your code work, most of the time it
won’t.
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java.util – Scanner, StringTokenizer
java.io – BufferedReader, FileReader,
PrintWriter
java.text – DecimalFormat
java.lang – String, Math
 Don’t need to import
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Reading flow in java:
a = a + 2;
‘A’ in ASCII is 65
ASCII is a subset of Unicode
Escape sequences
Compound operators
Increment/Decrement operators, pre/post
Documentation Reading
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Read up some common methods of the
following classes for use in the assignments
and exams:
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String
Scanner
Math
StringTokenizer
Character
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True/False
Multiple Choice
Code Tracing
Code Completion
Finding Errors
Other random kinds of questions like
matching exercises are a possibility
Exam Tips
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Read through all questions carefully
Write answers legibly
Show all work for partial credit
Do not spend too much time on any one
problem if it is causing trouble, come back to
it later on
Read through the chapter summaries in the
text book and all of Dr. S’s class notes
Good Luck!
char let=‘A’;
for (int i=1; i<=3; i++)
{ for(int j=i; j<5; j++)
System.out.print(let);
System.out.println();
let+=1;
}
Predict the output [6 points]
Difficulty: Medium
int n1=6, n2=10;
if(n1 >= n2/2) {
if(true && false)
System.out.print(“one”); }
else
System.out.print(“two”);
System.out.print(“three”);
Predict the output [3 points]
Difficulty: Easy
int num=6;
while(num<16) {
switch(num%4) {
case 1: System.out.println(“one”); break;
case 2: System.out.println(“two”);
case 3: System.out.println(“three”); break;
case 0: System.out.println(“multiple”);
}
num+=3;
}
Predict the output [4-6 points]
Difficulty: Medium
boolean a=true, b=false;
if (a && (true|| !b) )
if( b || (!a && false) )
System.out.println(“happy”);
else System.out.println(“ halloween”);
System.out.println(“goblins”);
Predict the output [6 points]
Difficulty: Medium
Sample Code Tracing #5
double a = 1.6;
int b = 13;
double c = (int) a * 2 + 1;
for (; b > 0; b-=3){
b /= c; // legal b/c equivalent to b = (int) (b/c);
System.out.println(b);
System.out.println(++b * 0.5 + b--);
}
Predict the output [9 points]
Difficulty: Hard
Sample Error Finding #1
String s = “hello world”;
String space = s.indexOf(“ “);
String pt2 = s.substring(space,s.length);
System.out.println(pt2);
Find the error [3 points]
Difficulty: Easy
Sample Error Finding #2
int a, b=10;
do {
if (b = 10)
a = 1.5; b = 10;
else if ((b-1)%2>5))
a = 3;
System.println(“a is ” + a “ and b is ” + b);
} while (a != 5)
Find the 8 errors [16 points]
Difficulty: 5 Medium, 3 Hard