生成器模式空指针异常

Question

当运行下面的代码时，我得到空指针异常:

public class Engine{
private String name = null;
private Mercedes m = null;
private Engine() {
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Mercedes getM() {
return m;
}
public void setM(Mercedes m) {
this.m = m;
}
public static EngineBuilder builder() {
return new EngineBuilder();
}
public static class EngineBuilder {
private Engine e = null;
public EngineBuilder builder() {
e = new Engine();
return this;
}
public Engine build() {
return this.e;
}
public EngineBuilder setName(String name) {
this.e.setName(name);
return this;
}
public EngineBuilder setM(Mercedes m) {
this.e.setM(m);
return this;
}
}
public static void main(String[] args) {
EngineBuilder builder = Engine.builder();
builder.setName("test");

Engine e = builder.build();
}
}
}

我期望Builder模式可以工作，但是我得到

线程"main"异常java.lang.NullPointerException:无法调用Engine.setName(String)"因为"this.e"null"

Answer 1

在你的代码中，EngineBuilder类只有默认构造函数，它不初始化Engine Object。写一个构造函数来初始化需要的对象。

Answer 2

EngineBuilder中的Engine e属性仅在其从未被调用的builder()方法中初始化。有多种方法来修复您的代码，尽管我将以不同的方式实现构建器模式:

public class Engine {
private final String name;
private Engine(Builder builder) {
this.name = builder.name;
}
public String getName() {
return name;
}
public static Builder builder() {
return new Builder();
}
@Override
public String toString() {
return name;
}
public static class Builder {
private String name;
private Builder() {
}
public Builder name(String name) {
this.name = name;
return this;
}
public Engine build() {
return new Engine(this);
}
}
public static void main(String[] args) {
Engine mercedes = Engine.builder().name("Mercedes").build();
System.out.println(mercedes);  // Mercedes
}
}

如果您希望Engine类的实例充当传统pojo，则从其属性中删除final，并为其添加public默认构造函数和设置器。

Answer 3

你需要在builder中添加构造函数。

public class Engine{
private String name = null;
private Mercedes m = null;
private Engine() {
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Mercedes getM() {
return m;
}
public void setM(Mercedes m) {
this.m = m;
}
public static EngineBuilder builder() {
return new EngineBuilder();
}
public static class EngineBuilder {
private Engine e = null;
// you need add Constructor in Builder
public EngineBuilder(){
e = new Engine();
}
public EngineBuilder builder() {
e = new Engine();
return this;
}
public Engine build() {
return this.e;
}
public EngineBuilder setName(String name) {
this.e.setName(name);
return this;
}
public EngineBuilder setM(Mercedes m) {
this.e.setM(m);
return this;
}
}
public static void main(String[] args) {
EngineBuilder builder = Engine.builder();
builder.setName("test");

Engine e = builder.build();
}
}