1､两个实例变量强制使用相同的T类型

通过使用类级别的通用参数T，Bar可以为两个实例变量强制使用相同的类型。

class Bar<T> {
    private Foo<T> var1;
    private Foo<T> var2;
    public Bar(Foo<T> var1, Foo<T> var2) {
        this.var1 = var1;
        this.var2 = var2;
    }
    public static void main(String[] args) {
        Foo<String> var1 = new Foo<>();
        Foo<Integer> var2 = new Foo<>();
        Bar<String> b = new Bar<>(var1, var2); 
    }
}

2､使用通配符不使用原始类型T

class Bar<T> {
    private Foo<? extends T> var1;
    private Foo<? extends T> var2;
    public Bar(Foo<? extends T> var1, Foo<? extends T> var2) {
        this.var1 = var1;
        this.var2 = var2;
    }
    public static void main(String[] args) {
        Foo<String> var1 = new Foo<>();
        Foo<Integer> var2 = new Foo<>();
        Bar<Object> b = new Bar<>(var1, var2); 
    }
}

或

class Bar<T> {
    private Foo<? super T> var1;
    private Foo<? super T> var2;
    public Bar(Foo<? super T> var1, Foo<? super T> var2) {
        this.var1 = var1;
        this.var2 = var2;
    }
    public static void main(String[] args) {
        Foo<Integer> var1 = new Foo<>();
        Foo<Number> var2 = new Foo<>();
        Bar<Integer> b = new Bar<>(var1, var2); 
    }
}