The English version of the summary of knowledge points covered by the questions：

The English version of the summary of knowledge points covered by the questions：

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1. The final Keyword and Constants

• When final is used on a variable, the variable becomes a constant and can only be assigned once.
• After it is assigned, it cannot be changed again. Otherwise, it causes a compile error.
• Constant names are usually written in all uppercase, such as MAX or PI.
• Example:

  final int MAX = 100;
  MAX = 200;   // Compile error: cannot assign a value to final variable
  

Common mistake:
If you see final followed by another assignment, the code will not compile.

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2. The + Operator and String Concatenation

• + has two meanings in Java:
  1. Between numbers: arithmetic addition;
  2. If at least one operand is a string: string concatenation.
• Expressions are evaluated from left to right.
• Once a string is encountered, everything after it is concatenated as a string, not added arithmetically.
• Examples:

  System.out.println("Sum:" + 5 + 3);   // Sum:53
  System.out.println("Hello" + 3 + 4);  // Hello34
  System.out.println(5 + 3 + "5");      // 85
  

• If you want the addition to happen first, use parentheses:

  System.out.println("Sum:" + (5 + 3)); // Sum:8
  

Common mistake:
Thinking "Sum:" + 5 + 3 will first compute 5 + 3, producing Sum:8.

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3. Integer Division and Modulo

• When two int values are divided, the result is still an int. The decimal part is truncated, not rounded.
• % gives the remainder.
• Examples:

  System.out.println(7 / 2);  // 3
  System.out.println(7 % 2);  // 1
  System.out.println(9 / 4);  // 2
  System.out.println(9 % 4);  // 1
  

• To get a decimal result, at least one operand must be a double:

  System.out.println(7 / 2.0);        // 3.5
  System.out.println((double) 7 / 2); // 3.5
  

Common mistake:
Thinking 7 / 2 is 3.5.

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4. Operator Precedence and Mixed Expressions

• Precedence: *, /, and % are higher than + and -.
• Operators with the same precedence are evaluated left to right.
• However, string concatenation changes how later operations are evaluated.
• Examples:

  int a = 6, b = 4;
  System.out.println(a + b + " " + a * b);
  // First a + b = 10, then a * b = 24, then concatenate: 10 24
  

  int a = 3, b = 4;
  System.out.println(a * b + " " + a + b);
  // First a * b = 12, then after the string, a and b are concatenated: 12 34
  

Common mistake:
Forgetting that after a string appears, later + operations become concatenation.

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5. Type Conversion

1. Automatic Type Conversion (Widening)

• A smaller type goes to a larger type. This is safe and does not require a cast.
• Example:

  int a = 5;
  double b = a;   // automatic conversion, b = 5.0
  

2. Explicit Casting (Narrowing)

• A larger type goes to a smaller type. An explicit cast is required.
• The decimal part is truncated, which may lose precision or cause overflow.
• Example:

  double d = 9.99;
  int x = (int) d;   // x = 9
  

• The following code causes a compile error:

  int x = 9.99;      // Compile error: possible lossy conversion from double to int
  int x = d;         // Compile error: double cannot be directly assigned to int
  

Common mistakes:
Forgetting to write (int), which causes a compile error; or thinking (int) 9.99 rounds to 10, when it actually truncates to 9.

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6. Three Common Types of Errors

Error Type	Characteristics	Example
Compile error	The code violates syntax or type rules and cannot compile	Reassigning a final variable; assigning a double directly to an int
Runtime error	The code compiles but crashes while running	Division by zero, array index out of bounds
Logic error	The code runs but produces incorrect results	Wrong formula, wrong operator

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7. Output and Newlines

• System.out.println(): prints and then moves to a new line.
• System.out.print(): prints without moving to a new line.
• \n can also represent a newline.
• Examples:

  System.out.print("A");
  System.out.print("B");   // Output: AB
  

  System.out.println("A");
  System.out.println("B"); // A and B are on separate lines
  

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8. Review Tips

1. When you see final, check whether it is assigned a second time.
2. When you see + with strings, evaluate from left to right and notice where the string appears.
3. When you see int / int, the result must be an integer; the decimal part is truncated.
4. When you see double being converted to int, an explicit (int) cast is required.
5. When you see output statements, pay attention to the difference between print and println.

These are the core Java knowledge points behind the questions you missed.