英文:
Java generics - Why is this assignment in the constructor illegal?
问题
为什么我在这段代码中得到编译器错误？如何修复它？
public class Container<T> {
private T content;
private T defaultValue;
public <T> Container(T defaultValue){
// 编译器错误 - 不兼容的类型：无法将 T 转换为 T。
this.defaultValue = defaultValue;
}
}
经过类型擦除，T defaultValue 会变成 Object defaultValue。那么为什么我们不能将 T defaultValue 赋值给 Object defaultValue？毕竟，在 Java 中每个类型都是一个对象。
英文:
Why do I get a compiler error in this code ? How do I fix it ?
public class Container<T> {
private T content;
private T defaultValue;
public <T> Container(T defaultValue){
//Compiler error - incompatible types: T cannot be converted to T.
this.defaultValue = defaultValue;
}
}
After type erasure, T defaultValue would become Object defaultValue. Then why can't we assign T defaultValue to Object defaultValue ? After all, every type in Java is an Object.
答案1
得分: 6
从构造函数前面删除<T>。Java认为您正在尝试创建一个新的泛型。它认为您在类语句中拥有的T与您在构造函数中拥有的T不同。在Java的视觉中，您有T1和T2。您试图将T2分配给T1变量。即使它们在方法、继承等方面可能相同，它们仍然是两个不同的泛型。
这是Java解释您所写内容的方式。
public class Container<T1> {
private T1 content;
private T1 defaultValue;
public <T2> Container(T2 defaultValue){
//编译器错误 - 不兼容的类型：无法将T转换为T。
this.defaultValue = defaultValue;
}
}
您原本想要写的是这样的。您不需要在任何地方指定< >内部的T，因为它已包含在类语法中。
public class Container<T> {
private T content;
private T defaultValue;
public Container(T defaultValue){
this.defaultValue = defaultValue;
}
}
英文:
Remove the <T> from in front of the constructor. Java thinks you are trying to create a new generic. It thinks the T you have in the class statement is a different T then you have in the constructor. In Java's eyes you have T1 and T2. You are trying to assign T2 to a T1 variables. Even though they may be identical in methods, inheritance, etc... they are two distinct generics.
This is how Java interprets what you've written.
public class Container<T1> {
private T1 content;
private T1 defaultValue;
public <T2> Container(T2 defaultValue){
//Compiler error - incompatible types: T cannot be converted to T.
this.defaultValue = defaultValue;
}
}
What you meant to write was this. You don't need to specify T inside < > anywhere since it's included in the class syntax.
public class Container<T> {
private T content;
private T defaultValue;
public Container(T defaultValue){
this.defaultValue = defaultValue;
}
}
答案2
得分: 2
编译器错误产生的原因是构造函数中的<T>。<T>的存在意味着该方法期望一个新的泛型参数T，而这个泛型参数与类所期望的参数不同。从方法签名中移除<T>，你的程序将会编译通过。
英文:
The reason that the compiler error is produced is because of the <T> in the constructor.
The presence of <T> means that the method is expecting a new generic parameter T. A generic parameter that is different from the one the class expects. Remove the <T> in the method signature and your program will compile.
答案3
得分: 0
public <T> Container(T defaultValue) {
// 表示这个方法会返回一个值，但构造函数不能显式返回任何值。
}
// 你可以这样写：
Container(T content, T defaultValue) {
// 这是完整的示例。
public class Test {
public static void main(String[] args) {
Container<String> container = new Container<>("value1", "value2");
}
}
class Container<T> {
private T content;
private T defaultValue;
Container(T content, T defaultValue) {
this.content = content;
this.defaultValue = defaultValue;
}
}
}
英文:
public <T> Container(T defaultValue){ means that it returns value but constructors can not return any value explicitly.
you can write:
Container(T content, T defaultValue) {
this is full example.
public class Test {
public static void main(String [] args) {
Container<String> container = new Container<>("value1","value2");
}
}
class Container <T> {
private T content;
private T defaultValue;
Container(T content, T defaultValue) {
this.content=content;
this.defaultValue =defaultValue;
}
}
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