- 提供一个接口 Measurable，它有一个方法 double getMeasure()，该方法以某种方式测量对象。让 Employee 类实现 Measurable 接口。提供一个方法 double average(Measurable[] objects)，该方法计算测量的平均值。使用它来计算一组 employee 的平均工资。
参考答案：
Measurable.java 的代码如下：
package com.example;
/**
* Created by toby on 17-12-5.
*/
public interface Measurable {
double getMeasure();
}
LearnJava.java 的代码如下：
package com.example;
public class LearnJava {
static class Employee implements Measurable {
private double salary = 0;
Employee() {
this(0);
}
Employee(double salary) {
this.salary = salary;
}
@Override
public double getMeasure() {
return salary;
}
}
static double average(Measurable[] objects) {
double sum = 0;
if (objects.length > 0) {
for (Measurable m : objects) {
sum += m.getMeasure();
}
return sum / objects.length;
}
return 0;
}
public static void main(String[] args) {
Measurable[] objects = new Measurable[50];
for (int i=1; i<=50; ++i) {
Employee employee = new Employee(i);
objects[i-1] = employee;
}
double result = average(objects);
System.out.println(result);
}
}
上述代码的运行结果：
01题运行结果
- 继续上一个练习并提供一个Measurable largest(Measurable[] objects) 方法。用该方法查找工资最高的员工的姓名。为什么需要强制转换？
参考答案：
package com.example;
public class LearnJava {
static class Employee implements Measurable {
private double salary = 0;
private String name = "";
Employee() {
this(0);
}
Employee(double salary) {
this(salary, "");
}
Employee(double salary, String name) {
this.salary = salary;
this.name = name;
}
@Override
public double getMeasure() {
return salary;
}
public String getName() {
return name;
}
}
static double average(Measurable[] objects) {
double sum = 0;
if (objects.length > 0) {
for (Measurable m : objects) {
sum += m.getMeasure();
}
return sum / objects.length;
}
return 0;
}
static Measurable largest(Measurable[] objects) {
Measurable maxMeasurable = null;
double max = 0;
if (objects.length > 0) {
for (Measurable m : objects) {
if (m.getMeasure() > max) {
max = m.getMeasure();
maxMeasurable = m;
}
}
}
return maxMeasurable;
}
public static void main(String[] args) {
Measurable[] objects = new Measurable[50];
for (int i=1; i<=50; ++i) {
Employee employee = new Employee(i, "test" + i);
objects[i-1] = employee;
}
Employee result = (Employee)largest(objects);
System.out.println(result.getName());
}
}
输出结果：
02题输出结果
之所以需要转换类型，是因为我们需要调用子类的方法，而我们返回的是被继承的接口的类型，该接口类型没有相应的方法。
- String 的所有父类有哪些？Scanner 类的呢？ImageOutputStream 类的呢？注意，每个类型都有自己的父类。没有声明父类的类或者接口，其父类为 Object
参考答案：
String 的所有父类如下：
String 的所有父类
Scanner 类的所有父类如下：
Scanner 类的所有父类
其中 Closeable 继承自 AutoCloseable
ImageOutputStream 类的所有父类如下：
ImageOutputStream 类的所有父类
上图中 ImageInputStream 的所有父类如下：
ImageInputStream 的所有父类
其中 Closeable 继承自 AutoCloseable
Closeable 继承自 AutoCloseable
- 实现 IntSequence 类的一个静态方法，该方法使用参数产生序列。例如，IntSequence.of(3, 1, 4, 1, 5, 9) 产生有六个值的序列。如果你返回匿名内部类的实例，则额外加分。
参考答案：
IntSequence 接口的实现如下：
package com.example;
import java.util.ArrayList;
/**
* Created by toby on 17-12-5.
*/
public interface IntSequence {
public static IntSequence of(int... ints) {
return new DigitSequence(ints);
}
static class DigitSequence implements IntSequence {
private ArrayList integers = new ArrayList<>();
public DigitSequence(int[] ints) {
integers.clear();
integers = null;
integers = new ArrayList<>(ints.length);
for (int anInt : ints) {
integers.add(anInt);
}
}
public boolean hasNext() {
return integers.size() > 0;
}
public int next() {
int v = integers.get(0);
integers.remove(0);
return v;
}
}
}
测试使用的代码如下：
package com.example;
public class LearnJava {
public static void main(String[] args) {
IntSequence.DigitSequence digits = (IntSequence.DigitSequence)
IntSequence.of(3, 1, 4, 1, 5, 9);
while (digits.hasNext()) {
System.out.println(digits.next());
}
}
}
输出结果：
04题输出结果
- 实现 IntSequence 类的静态 constant 方法，该方法产生无限常量序列。例如，IntSequence.constant(1) 产生值 1 1 1...永无休止。如果你使用 lambda 表达式，则额外加分。
IntSequence 类的代码如下：
package com.syberos.learnjava;
import java.util.ArrayList;
/**
* Created by toby on 17-12-5.
*/
public interface IntSequence {
public static IntSequence of(int... ints) {
return new DigitSequence(ints);
}
public static IntSequence constant(int i) {
return new InfiniteSequence(i);
}
static class DigitSequence implements IntSequence {
private ArrayList integers = new ArrayList<>();
public DigitSequence(int[] ints) {
integers.clear();
integers = null;
integers = new ArrayList<>(ints.length);
for (int anInt : ints) {
integers.add(anInt);
}
}
public boolean hasNext() {
return integers.size() > 0;
}
public int next() {
int v = integers.get(0);
integers.remove(0);
return v;
}
}
static class InfiniteSequence implements IntSequence {
private int constant = 0;
public InfiniteSequence(int i) {
constant = i;
}
public boolean hasNext() {
return true;
}
public int next() {
return constant;
}
}
}
测试用的代码如下：
package com.syberos.learnjava;
public class LearnJava {
private static void printSequence(IntSequence.InfiniteSequence ints) {
Runnable runnable = () -> {
while (ints.hasNext()) {
System.out.println(ints.next());
}
};
new Thread(runnable).start();
}
public static void main(String[] args) {
IntSequence.InfiniteSequence ints = (IntSequence.InfiniteSequence)
IntSequence.constant(1);
printSequence(ints);
}
}