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--- j1-1_strings [2024/11/28 00:47] – appledog
+++ j1-1_strings [2024/12/05 06:40] (current) – appledog
@@ Line 10: / Line 10: @@
 Scroll down to the second option, the "Community Edition", which is free.
-== Entering The Program
+== Variables
+First explain the basic structure of a Java program and demonstrate basic integer variables. Then show how to print them out.
+<Code:Java>
+public class Main {
+    public static void main(String[] args) {
+        int a = 5;
+        int b = 3;
+        int c = a + b;
+        System.out.println(c);
+    }
+}
+</Code>
+Don't discuss anything else yet, just play around with integer math a bit, and show what a function is:
+<Code:Java>
+public class Main {
+    public static void main(String[] args) {
+        int a = 5;
+        int b = 3;
+        int c = a + b;
+        System.out.println(c);
+        int d = mul(a, b);
+        System.out.println(d);
+    }
+    public static int mul(int a, int b) {
+        int c = a * b;
+        return c;
+    }
+}
+</Code>
+== Strings
 Start a new project called "J1-1 Strings". Then type the following code into the Main.java file:
 === Main.java
 <Code:Java|Main.java>
+import java.util.Scanner;
 public class Main {
     public static void main(String[] args) {
@@ Line 54: / Line 95: @@
 Teacher may discuss the answers here.
-== The Code
+== Don't Read This Section
 Here's an explanation of what each line of the program does. We will look at it like a computer looks at it. First, here's the basic starter program:
@@ Line 92: / Line 133: @@
 This means the function doesn't return anything. Actually this surprises me, since it seems a main function should return an error code. But no, in Java there is no return. Sounds ominous. Try changing the void to an int and returning a 5. It doesn't work.
-== The Code
+== The Code (Read This Part!)
 === System.out.println("Hello!")
 This kind of code just prints a message out.
@@ Line 128: / Line 169: @@
 <Code:Java|charAt()>
-    String a = "cars";
+    String s = "Appledog";
-    char secondChar = a.charAt(1);        // Access the 2nd character (index 1)
+    char c = s.charAt(4);        // Access the 5th character (index starts at 0)
-    System.out.println("The second character is: " + secondChar);
+    System.out.println("The fifth character is: " + c);
 </Code>
+=== Strings are immutable
+You cannot deal with a string like you can in Python, by directly indexing it. You need to first convert it into an array, and then back again.
+<Code:Java|string as array>
+    String s = "Appledog";
+    // Convert String to char[]
+    char[] a = s.toCharArray();
+    // Modify the array
+    a[5] = 'b';
+    // Convert char[] back to String
+    s = new String(a);
+    System.out.println(s);
+</Code>
+This prints "Applebog".
+Here concludes the lesson. Next: [[J1-2 if-else]]