AP CSA — Unit 3: Class Creation

Draft v2 — 15 AP-style MCQs at exam level (3 MED + 12 HARD); class/constructor/method trace heavy with realistic distractors. HARDs upgraded: aliasing + compound assignment, static aggregate w/ mutator, reassign-vs-mutate ref param, mutator order w/ conditional cap, fluent interface, static seq + reassignment-through-aliasing. Assumes top-score target. No calculator. Java Quick Reference assumed available.v2 草稿 —— 15 道 AP 风格选择题（MC），考试难度（3 道 MED + 12 道 HARD）；偏重类/构造方法/方法追踪（class / constructor / method），干扰项贴近真题。HARD 题目升级方向：别名（aliasing）+ 复合赋值（compound assignment）、static 聚合配合实例修改器（mutator）、引用参数的"重新赋值 vs 就地修改"、带条件上限的修改器调用顺序、链式接口（fluent interface）、static 自增序列 + 别名重新赋值。面向冲击 5 分的考生。不允许使用计算器。假定考生可使用 Java Quick Reference。

Multiple Choice选择题（Multiple Choice）

Choose the best of the four options. Assume all referenced classes and methods are imported / available unless otherwise stated. "Consider the following code segment" implies the code compiles and runs without exception unless the question states otherwise.从四个选项中选择最合适的一个。除非另有说明，假定所有引用的类（class）与方法（method）均已导入且可用。"Consider the following code segment"（"考虑以下代码段"）默认代码可以正常编译并运行（不抛出异常），除非题目另有说明。

Q1MEDIUM AP MC 3.9 this Keyword [1]

Consider the following constructor.考虑以下构造方法（constructor）。

public class Box {
  private int size;
  public Box(int size) {
    this.size = size;
  }
}

What does the statement this.size = size; do?语句 this.size = size; 的作用是什么？

(A) Assigns the value of the parameter size to the instance variable size.将参数 size 的值赋给实例变量（instance variable） size。
(B) Assigns the parameter size to itself; the instance variable is unaffected.把参数 size 赋给它自己；实例变量不受影响。
(C) Causes a compile-time error because the parameter and instance variable have the same name.出现编译错误，因为参数与实例变量同名。
(D) Creates a new local variable named size that is discarded.创建一个名为 size 的新局部变量然后被丢弃。

Q2MEDIUM AP MC 3.7 static Variables [1]

Which of the following statements about a static variable declared in a Java class is true?下列关于 Java 类中 static 变量的描述，哪一项是正确的？

(A) Each instance of the class has its own independent copy.该类的每个实例都拥有各自独立的一份。
(B) It is automatically initialized to null regardless of its declared type.无论声明类型为何，都会被自动初始化为 null。
(C) A single copy is shared by all instances of the class.所有实例共享同一份变量（类级别变量 class-level variable）。
(D) It can only be read, never modified, after initialization.初始化后只能被读取、不能被修改。

Q3MEDIUM AP MC 3.4 Default Constructor [1]

Consider a Java class that declares no constructors of its own. Which of the following is true?假设某个 Java 类没有声明任何自定义的构造方法（constructor）。下列哪一项是正确的？

(A) The class cannot be instantiated.该类无法被实例化。
(B) The compiler provides a no-argument default constructor with an empty body.编译器会自动提供一个无参、方法体为空的默认构造方法（default constructor）。
(C) The class will fail to compile.该类无法通过编译。
(D) The class can only be used as a static utility class.该类只能用作静态工具类。

Q4HARD AP MC 3.6 Aliasing + compound assignment [1]

Consider the following class and code.考虑以下类与代码。

public class Box {
  public int n;
  public Box(int n) { this.n = n; }
}

// in main:
Box a = new Box(5);
Box b = a;
b.n = 10;
a.n += b.n;
System.out.println(a.n + " " + b.n);

What is printed?输出是什么？（注意 b = a; 之后 a 与 b 是别名 aliasing，指向同一个对象。）

(A) 5 10
(B) 15 10
(C) 20 10
(D) 20 20

Q5HARD AP MC 3.7 static aggregate + instance mutator [1]

Consider the following class.考虑以下类。

public class Sensor {
  private static int total = 0;
  private int reading;
  public Sensor(int r) { reading = r; total += r; }
  public void update(int r) {
    total -= reading;
    reading = r;
    total += r;
  }
  public static int total() { return total; }
}

// in main:
Sensor s1 = new Sensor(10);
Sensor s2 = new Sensor(20);
s1.update(50);
s2.update(0);
System.out.println(Sensor.total());

What is printed?输出是什么？（注意 static 聚合变量与实例修改器 mutator 在每次 update 中如何维持总和。）

(A) 30
(B) 50
(C) 70
(D) 80

Q6HARD AP MC 3.8 Missing this. Trap [1]

Consider the following class.考虑以下类。

public class Point {
  private int x;
  public Point(int x) {
    x = x;        // note: no this.
  }
  public int getX() { return x; }
}

// in main:
Point p = new Point(7);
System.out.println(p.getX());

What is printed (or what happens)?输出是什么（或会发生什么）？（注意构造方法中没有写 this.。）

(A) 0
(B) 7
(C) -1
(D) A compile-time error: self-assignment is not allowed.编译错误：自赋值不被允许。

Q7HARD AP MC 3.6 Reassigning a ref param vs mutating [1]

Consider the following class and methods.考虑以下类与方法。

public class Box {
  public int n;
  public Box(int n) { this.n = n; }
}

public static void replace(Box b) {
  b = new Box(99);
}

public static void mutate(Box b) {
  b.n = 99;
}

// in main:
Box one = new Box(1);
Box two = new Box(2);
replace(one);
mutate(two);
System.out.println(one.n + " " + two.n);

What is printed?输出是什么？（关键区别：replace 重新赋值本地引用 reassign reference，mutate 通过引用就地修改对象 mutate via reference。）

(A) 1 2
(B) 1 99
(C) 99 99
(D) 99 2

Q8HARD AP MC 3.5 Mutator order with conditional cap [1]

Consider the following class.考虑以下类。

public class Score {
  private int v;
  public Score(int v) { this.v = v; }
  public void add(int n) { v += n; }
  public void cap(int max) { if (v > max) v = max; }
  public int get() { return v; }
}

Score s = new Score(10);
s.add(20);
s.cap(15);
s.add(5);
s.cap(50);
System.out.println(s.get());

What is printed?输出是什么？（按顺序逐步执行每个修改器调用，注意 cap 仅当超过 max 时才裁剪。）

(A) 15
(B) 20
(C) 30
(D) 35

Q9HARD AP MC 3.5 Fluent interface (return this) [1]

Consider the following class.考虑以下类。

public class Counter {
  private int n;
  public Counter()       { n = 0; }
  public Counter inc()   { n++; return this; }
  public Counter dec()   { n--; return this; }
  public int peek()      { return n; }
}

Counter c = new Counter();
int x = c.inc().inc().inc().dec().peek();
System.out.println(x + " " + c.peek());

What is printed?输出是什么？（每个修改器 return this;，构成链式接口 fluent interface，所有方法调用作用于同一个对象。）

(A) 0 2
(B) 2 0
(C) 2 2
(D) 4 4

Q10HARD AP MC 3.6 Object Equality (==) [1]

Consider the following class and code.考虑以下类与代码。

public class Pt {
  public int x, y;
  public Pt(int x, int y) { this.x = x; this.y = y; }
}

Pt p = new Pt(1, 2);
Pt q = new Pt(1, 2);
Pt r = p;
System.out.println((p == q) + " " + (p == r));

What is printed?输出是什么？（提示：== 比较对象的引用地址 reference equality，而不是字段值。）

(A) true true
(B) false true
(C) true false
(D) false false

Q11HARD AP MC 3.8 Local Variable Shadows [1]

Consider the following class.考虑以下类。

public class Tank {
  private int level = 100;
  public void drain() {
    int level = 0;     // local declaration
    level -= 10;
  }
  public int getLevel() { return level; }
}

Tank t = new Tank();
t.drain();
System.out.println(t.getLevel());

What is printed?输出是什么？（注意 drain 中重新声明了局部 level，会"遮蔽" shadow 同名实例变量。）

(A) 90
(B) 100
(C) -10
(D) 0

Q12HARD AP MC 3.7 static Accessing Instance [1]

Consider the following class declaration.考虑以下类的声明。

public class Counter {
  private int count = 0;
  public static int getCount() {
    return count;
  }
}

What is the result of attempting to compile and run this class?尝试编译并运行此类的结果是什么？

(A) It compiles and works correctly.能正确编译并运行。
(B) It compiles, but getCount() always returns 0.能编译，但 getCount() 永远返回 0。
(C) A compile-time error: a non-static field cannot be referenced from a static context.编译错误：非静态字段无法从静态上下文（static context）中访问。
(D) A run-time error: NullPointerException on the call to getCount().运行时错误：调用 getCount() 时抛出 NullPointerException。

Q13HARD AP MC 3.3 Encapsulation Access [1]

Consider the following class.考虑以下类。

public class Person {
  private String name;
  public Person(String name) { this.name = name; }
}

From a different class in the same package, a programmer writes:在同一个包中的另一个类里，某程序员写下：

Person p = new Person("Alice");
p.name = "Bob";   // line X

What happens at line X?在 line X 处会发生什么？（提示：private 字段的可见性 visibility。）

(A) It compiles; p.name is now "Bob".能通过编译；p.name 现在为 "Bob"。
(B) It compiles, but the assignment is silently ignored.能通过编译，但赋值被悄悄忽略。
(C) A compile-time error: name has private access in Person.编译错误：name 在 Person 中是私有访问（private access）。
(D) A run-time IllegalAccessException is thrown.运行时抛出 IllegalAccessException。

Q14HARD AP MC 3.6 Primitive vs Object Params [1]

Consider the following class and code.考虑以下类与代码。

public class Wrap {
  public int n;
  public Wrap(int n) { this.n = n; }
}

public static void modify(int x, Wrap w) {
  x   = 999;
  w.n = 999;
}

// in main:
int x = 5;
Wrap w = new Wrap(5);
modify(x, w);
System.out.println(x + " " + w.n);

What is printed?输出是什么？（提示：基本类型按值传递 pass by value，对象引用传的是地址副本——可以通过它修改对象，但重新赋值给参数本身不影响调用方。）

(A) 5 5
(B) 999 999
(C) 5 999
(D) 999 5

Q15HARD AP MC 3.7 static seq + aliasing + toString [1]

Consider the following class and code.考虑以下类与代码。

public class Tag {
  private static int seq = 0;
  private String label;
  private int id;
  public Tag(String label) {
    this.label = label;
    this.id = ++seq;
  }
  public String toString() { return "#" + id + ":" + label; }
}

// in main:
Tag a = new Tag("alpha");
Tag b = new Tag("beta");
Tag c = a;
a = new Tag("alpha");
System.out.println(a + " " + b + " " + c);

What is printed?输出是什么？（注意每个构造方法都会预增 ++seq，然后注意 c = a; 此时 a 仍然指向哪个对象。）

(A) #1:alpha #2:beta #1:alpha
(B) #3:alpha #2:beta #3:alpha
(C) #3:alpha #2:beta #1:alpha
(D) #1:alpha #2:beta #3:alpha

Class Creation类的创建

Multiple Choice选择题（`Multiple Choice`）

Class Creation类的创建

Multiple Choice选择题（Multiple Choice）

Multiple Choice选择题（`Multiple Choice`）