Java泛型+构建器模式
Java泛型+构建器模式
提问于 2010-07-08 22:27:53
如何调用下面的start()？
package com.example.test;
class Bar {}
public class Foo<K>
{
final private int count;
final private K key;
Foo(Builder<K> b)
{
this.count = b.count;
this.key = b.key;
}
public static class Builder<K2>
{
int count;
K2 key;
private Builder() {}
static public <K3> Builder<K3> start() { return new Builder<K3>(); }
public Builder<K2> setCount(int count) { this.count = count; return this; }
public Builder<K2> setKey(K2 key) { this.key = key; return this; }
public Foo<K2> build() { return new Foo(this); }
}
public static void main(String[] args)
{
Bar bar = new Bar();
Foo<Bar> foo1 = Foo.Builder.start().setCount(1).setKey(bar).build();
// Type mismatch: cannot convert from Foo<Object> to Foo<Bar>
Foo<Bar> foo2 = Foo.Builder<Bar>.start().setCount(1).setKey(bar).build();
// Multiple markers at this line
// - Bar cannot be resolved
// - Foo.Builder cannot be resolved
// - Syntax error on token ".", delete this token
// - The method start() is undefined for the type Foo<K>
// - Duplicate local variable fooType mismatch: cannot convert from Foo<Object> to Foo<Bar>
Foo<Bar> foo3 = Foo<Bar>.Builder.start().setCount(1).setKey(bar).build();
// Multiple markers at this line
// - Foo cannot be resolved
// - Syntax error on token ".", delete this token
// - Bar cannot be resolved
}
}回答 3
Stack Overflow用户
回答已采纳
发布于 2010-07-08 22:33:02
你很接近了：
Foo.Builder.<Bar> start().setCount(1).setKey(bar).build();干杯！:)
如果编译器不能自己推断出方法的类型参数，你可以通过调用obj.<Type> method(...)来强制它。
附注:您可能想要使用：
public Foo<K2> build() {
return new Foo<K2>(this);
}避免使用原始类型。
Stack Overflow用户
发布于 2010-07-08 22:40:11
Andrei的方法还可以，但大多数程序员可能会为这种相当未知的语法而苦苦挣扎。使用下面的方法可能更容易：
static public <K3> Builder<K3> start(Class<K3> cls) { return new Builder<K3>(); }
Foo<Bar> foo1 = Foo.Builder.start(Bar.class).setCount(1).setKey(bar).build();传递该类只是为了帮助处理泛型类型。它并不美观，但至少语法是常识。
另一种选择是立即从泛型类型的对象开始：
Foo<Bar> foo1 = Foo.Builder.startWithKey(bar).setCount(1).build();Stack Overflow用户
发布于 2018-10-13 22:50:04
下面是我会怎么做：
package odmor2018.krit.rtti.builderpattern;
import java.lang.reflect.Field;
import java.util.Arrays;
import java.util.List;
import java.util.logging.Level;
import java.util.logging.Logger;
import java.util.stream.Collectors;
public class Person {
private String firstName;
private String middleName;
private String lastName;
private boolean sex;
public Person(String firstName, String middleName, String lastName, boolean sex) {
this.firstName = firstName;
this.middleName = middleName;
this.lastName = lastName;
this.sex = sex;
}
public Person() {
}
@Override
public String toString() {
return "Person{" + "firstName=" + firstName + ", middleName=" + middleName + ", lastName=" + lastName + ", sex=" + sex + '}';
}
public static class Builder {
private final Field[] fields = Person.class.getDeclaredFields();
private final List<Field> fieldsList = Arrays.asList(fields);
private final List<String> fNames = fieldsList.stream().map(f -> f.getName()).collect(Collectors.toList());
private final Person nP = new Person();
public Builder with(String fName, Object value) {
if (fNames.contains(fName)) {
int fInd = fNames.indexOf(fName);
try {
Field f = fields[fInd];
f.setAccessible(true);
f.set(nP, value);
} catch (Exception ex) {
Logger.getLogger(Person.class.getName()).log(Level.SEVERE, null, ex);
}
}
return this;
}
public Person createPerson2() {
return nP;
}
}
public static void main(String[] args) {
Person p3 = new Person.Builder()
.with("firstName", "doooobri2")
.with("sex", false)
.createPerson2();
System.err.println("p3:" + p3);
}
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/3204623
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