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 vs Python: Strings

String s1 = "Maelle";
String s2 = "";

s1 = "Maelle"
s2 = ''

Strings

• Strings are a sequence of characters
• Characters are separate in Java

  • Denoted with single ('') quotes

  • Only a single character allowed inside

    char c1 = 'j';
    char c2 = 'H';
    char c3 = 'he'; // ERROR

String Concatenation

• Strings can be concatenated(combined) using the + operator

void main() {
    String s1 = "Hello";
    String s2 = "World";

    String s3 = s1 + s2;

    IO.println(s3);            // Prints: HelloWorld
    IO.println(s1 + " " + s2); // Prints: Hello World
}

Plus Again?

void main() {
    int a = 5;
    int b = 7;
    IO.println(a + b);               // Prints: 12
    IO.println("Sum is " + a + b);   // Prints: Sum is 57
    IO.println("Sum is " + (a + b)); // Prints: Sum is 12
    IO.println(a + b + "boo");       // Prints: 12boo
}

PEMDAS

• Expressions in Java generally follow the order of operations you learned in school, PEMDAS
  • Parenthesis before
  • Exponentiation before
  • Multiplication and division before
  • Addition and subtraction

void main() {
    int a = 3;
    a = 2 * a + 1;
    int b = 2;
    IO.println(a + "/" + b + "= " + a / b);
}

Methods

• Methods are functions we can call on a given variable with a type such as a String

• We call methods using the following syntax:

  <<variableName>>.<<methodName>>(<<argList>>);

• For example, we can check if two Strings are equal using the equal method. We cannot use the == operator on Strings to check for equality:

  void main() {
      String s1 = "CS152";
      String s2 = "CS";
      s2 += "152";
      IO.println(s1.equals(s2)); // true
      IO.println(s1 == s2);      // false
  }

== vs equals

• == is used to check for equality of primitive types
  • This includes:
    • int, char, boolean, double, etc.
    • Primitive types always start with a lower case letter
    • These are types which are baked into the language
• equal is used to check for equality of reference types
  • This includes:
    • Strings, Arrays, Lists, etc.
    • Reference types usually start with an upper case letter
    • These are types which are defined in the language itself

String Methods: Length

• We can determine the length of a string using the length method:

void main() {
    String s1 = "Maelle";
    String s2 = "Verso";
    IO.println(s1.length() + s2.length()); // Prints: 11
}

String Methods: charAt

• We can get the character at a given index using the charAt method.
• You must be careful to make sure the index exists

void main() {
    String s1 = "Maelle";
    IO.println(s1.charAt(0)); // Prints: M
    IO.println(s1.charAt(5)); // Prints: e
    IO.println(s1.charAt(6)); // ERROR
}

String Methods: toUpperCase

• We can force all characters in a string to be upper case using the toUpperCase method
• Note that this function returns a new string which is upper cased
• It leaves the original string unchanged!

void main() {
    String s1 = "Maelle";
    s1.toUpperCase();
    IO.println(s1); // Prints: Maelle
    s1 = s1.toUpperCase();
    IO.println(s1); // Prints: MAELLE
}

String Methods: substring

• We can extract a substring from a string using the substring method
• The substring method given one argument means it starts at the index we give it and goes to the end of the string
• The substring method given two arguments means it starts at the index we give it and goes up to but not including the second index

void main() {
    String s1 = "Maelle";
    IO.println(s1.substring(2));    // Prints: elle
    IO.println(s1.substring(3, 5)); // Prints: ll
}

Looping Over Strings

• We can loop over strings using the charAt function
• For example we could print out all the characters at even indexes:

void main() {
    String s1 = "expedition33";
    for (int i = 0; i < s1.length(); i++) {
        if (i % 2 == 0) {
            char c = s1.charAt(i);
            IO.println(c);
        }
    }
}

Creating Strings With Concatenation

• What if instead of printing out the individual even index characters we wanted to create a new string with them?

  void main() {
      String s1 = "expedition33";
      String result = "";
      for (int i = 0; i < s1.length(); i++) {
          if (i % 2 == 0) {
              char c = s1.charAt(i);
              result += c;
          }
      }
      IO.println(result);
  }

• IntelliJ may warn you do change this, don’t listen!