Lecture 03 Java Intro

Joseph Haugh

University of New Mexico

Free Recall

Get out a sheet of paper or open a text editor
For 2 minutes write down whatever comes to mind about the last class
- This could be topics you learned
- Questions you had
- Connections you made

Java Intro

Let’s start by comparing the syntax of the prerequisite topics in Python vs Java

Comments

Comments are lines of code which are only used to communicate to programmers
Comments do not effect the output of a program
Can add more context to a program

Python

# This is a comment
x = 5 # This is an inline comment

Java

// This is a comment
int x = 5; // This is an inline comment

Differences

Python

# This is a comment
x = 5 # This is an inline comment

Java

// This is a comment
int x = 5; // This is an inline comment

Java: all variables declarations must have an explicit type
Java: most statements must end with a semicolon(;)

Variables

Variables store information
This can be numbers, string, lists and more
Variables can have their values changed as the program runs

Python

x = 5     # x is 5
y = 6 + 3 # y is 9
z = y * 2 # z is 18

Java

int x = 5;     // x is 5
int y = 6 + 3; // y is 9
int z = y * 2; // z is 18

If Statements

Allows a program to take branching paths

Python

x = 5
if x > 5: 
    # If x is greater than 5
    # Then do these statements
    print("Greater than 5")
elif x == 5: 
    # Else if x is equal to 5
    # Then do these statements
    print("Equal to 5")
else:
    # Else do these statements
    print("Less than 5")

Java

int x = 5;
if (x > 5) { 
    // If x is greater than 5
    // Then do these statements
    IO.println("Greater than 5");
} else if (x == 5) { 
    // Else if x is equal to 5
    // Then do these statements
    IO.println("Equal to 5");
} else { 
    // Else do these statements
    IO.println("Less than 5");
}

Differences

Python

x = 5
if x > 5: 
    # If x is greater than 5
    # Then do these statements
    print("Greater than 5")
elif x == 5: 
    # Else if x is equal to 5
    # Then do these statements
    print("Equal to 5")
else:
    # Else do these statements
    print("Less than 5")

Java

int x = 5;
if (x > 5) { 
    // If x is greater than 5
    // Then do these statements
    IO.println("Greater than 5");
} else if (x == 5) { 
    // Else if x is equal to 5
    // Then do these statements
    IO.println("Equal to 5");
} else { 
    // Else do these statements
    IO.println("Less than 5");
}

Java: if conditions must be inside parenthesis
Java: no elif instead use else if
Java: Code inside if block is denoted using curly braces({})

Looping

Allows a program to repeat a sequence of statements many times
While loops repeat the body of the loop until the condition is false
Loops can repeat as many times as they want

Python

i = 0
while i < 5: 
    # Loop until i is NOT less than 5
    # While the condition holds do 
    # these statements
    print(i)
    i = i + 1
    # Then check the condition again 
    # and repeat if necessary

Java

int i = 0;
while (i < 5) {
    // Loop until i is NOT less than 5
    // While the condition holds do 
    // these statements
    IO.println(i);
    i = i + 1;
    // Then check the condition again 
    // and repeat if necessary
}

Indentation

In Python indentation matters
In Java indentation doesn’t matter
Instead curly braces ({}) are used to denote which statements are inside a block of code

Python

x = 5
y = 6
if x < 5:
    x = 7 # Inside

y = 8 # Outside
print(x) # Prints 5
print(y) # Prints 8

Java

int x = 5;
int y = 6;
if (x < 5) {
    x = 7; // Inside
}
y = 8; // Outside
IO.println(x); // Prints 5
IO.println(y); // Prints 8

Indentation

Python

x = 0
y = 0
while x < 10:
    x += 1   # Inside
    print(x) # Inside
print(y) # Outside

Java

int x = 0;
int y = 0;
while (x < 10) {
    x += 1;        // Inside
    IO.println(x); // Inside
}
IO.println(y); // Outside

Input

Getting user input in Python and Java is easy

Python

# Read an int from user input
x = int(input())
# Now you can use the number
if x >= 0:
    print("Positive")

else:
    print("Negative")

Java

// Read an int from user input
int x = Integer.parseInt(IO.readln());
// Now you can use the number
if (x >= 0) {
    IO.println("Positive");
}
else {
    IO.println("Negative");
}