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Java中要想自定义类的对象可以被复制，自定义类就必须实现Cloneable中的clone()方法，如下：
1 public class Student implements Cloneable { 2 3 private String name; 4 5 private int age; 6 7 private Professor professor; 8 9 public String getName() {10 return name;11 }12 13 public void setName(String name) {14 this.name = name;15 }16 17 public int getAge() {18 return age;19 }20 21 public void setAge(int age) {22 this.age = age;23 }24 25 public Professor getProfessor() {26 return professor;27 }28 29 public void setProfessor(Professor professor) {30 this.professor = professor;31 }32 33 @Override34 public String toString() {35 return "Student [name=" + name + ", age=" + age + ", professor="36 + professor + "]";37 }38 39 public Object clone() throws CloneNotSupportedException{40 return super.clone();41 }42 43 } 其中，Professor类同样为自定义类：
public class Professor { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return "Professor [name=" + name + ", age=" + age + "]"; }}
然而，当自定义类的字段的类型不是基本数据类型时，上面实现了clone()方法会导致问题，不信看下面的代码：
【程序实例1】
1 public class ShadowCopy { 2 3 public static void main(String[] args) { 4 Professor p1 = new Professor(); 5 p1.setName("Professor Zhang"); 6 p1.setAge(30); 7 8 Student s1 = new Student(); 9 s1.setName("xiao ming");10 s1.setAge(18);11 s1.setProfessor(p1);12 13 System.out.println(s1);14 15 try {16 Student s2 = (Student) s1.clone();17 Professor p2 = s2.getProfessor();18 p2.setName("Professor Li");19 p2.setAge(45);20 s2.setProfessor(p2);21 System.out.println("复制后的：s1 = " + s1);22 System.out.println("复制后的：s2 = " + s2);23 } catch (CloneNotSupportedException e) {24 e.printStackTrace();25 }26 27 }28 29 } 【运行结果1】
1 Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]2 复制后的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Li, age=45]]3 复制后的：s2 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Li, age=45]]
【结果分析】
学生s1的导师为30岁的Professor Zhang，恰好学生s2与学生s1同名同岁，但是s2的导师为45岁的Professor Li，于是我们顺理成章地复制复制s1并复制给s2，再修改下s2的导师的信息。可是，问题出现了，当我们修改了s2的导师后，s2的信息是对了，但是s1的导师信息也跟着修改了，这可不是我们期望的。
【问题分析】
程序实例1中的问题出在哪儿呢？我们已经对Student类实现了clone()方法，怎么还是出问题了呢？我们在看下面的代码：
【程序实例2】
1 public class ShadowCopy { 2 3 public static void main(String[] args) { 4 Professor p1 = new Professor(); 5 p1.setName("Professor Zhang"); 6 p1.setAge(30); 7 8 Student s1 = new Student(); 9 s1.setName("xiao ming");10 s1.setAge(18);11 s1.setProfessor(p1);12 13 System.out.println(s1);14 15 try {16 Student s2 = (Student) s1.clone();17 s2.setName("xiao hong");18 s2.setAge(17);19 Professor p2 = s2.getProfessor();20 p2.setName("Professor Li");21 p2.setAge(45);22 s2.setProfessor(p2);23 System.out.println("复制后的：s1 = " + s1);24 System.out.println("复制后的：s2 = " + s2);25 } catch (CloneNotSupportedException e) {26 e.printStackTrace();27 }28 29 }30 31 } 【运行结果】
1 Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]2 复制后的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Li, age=45]]3 复制后的：s2 = Student [name=xiao hong, age=17, professor=Professor [name=Professor Li, age=45]]
【结果分析】
这次，我们在clone后，又修改了s2的name和age，从结果可以看出，s1的name和age并没有因为s2的修改而改变。
结合程序实例1和程序实例2，我们发现Student的字段如果不是一个引用时，修改clone()得到对象的该字段(name, age)时并不会影响原来的对象，但是当字段为一个引用时，修改clone()得到对象的该字段(professor)时并会影响原来的对象。上面实现的clone()方法为浅复制（shadow copy）。
如果想要clone()得到的新对象的修改不会影响被复制的对象的字段时，我们就需要实现深复制（deep copy），代码修改如下：
1 public class Professor implements Cloneable { 2 3 private String name; 4 5 private int age; 6 7 public String getName() { 8 return name; 9 }10 11 public void setName(String name) {12 this.name = name;13 }14 15 public int getAge() {16 return age;17 }18 19 public void setAge(int age) {20 this.age = age;21 }22 23 @Override24 public String toString() {25 return "Professor [name=" + name + ", age=" + age + "]";26 }27 28 public Object clone() throws CloneNotSupportedException{29 return super.clone();30 }31 32 } public class Student implements Cloneable { private String name; private int age; private Professor professor; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public Professor getProfessor() { return professor; } public void setProfessor(Professor professor) { this.professor = professor; } @Override public String toString() { return "Student [name=" + name + ", age=" + age + ", professor=" + professor + "]"; } public Object clone() throws CloneNotSupportedException{ Student newStudent = (Student) super.clone(); newStudent.professor = (Professor) professor.clone(); return newStudent; }} 再次运行【程序实例2】得到的结果为：
1 Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]2 复制后的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]3 复制后的：s2 = Student [name=xiao hong, age=17, professor=Professor [name=Professor Li, age=45]]
可以看到：修改clone()得到的s2的任何字段都不会影响s1的字段，这也就是深复制的作用。
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