单选题1. import java.util.*; 2. class SubGen { 3. public static void main(String [] args) { 4. //insert code here 5. } 6. } class Alpha { } class Beta extends Alpha { } class Gamma extends Beta { } 和四段代码片段: s1. ArrayList〈? extends Alpha〉 list1 = new ArrayList〈Gamma〉(); s2. ArrayList〈Alpha〉 list2 = new ArrayList〈? extends Alpha〉(); s3. ArrayList〈? extends Alpha〉 list3 = new ArrayList〈? extends Beta〉(); s4. ArrayList〈? extends Beta〉 list4 = new ArrayList〈Gamma〉(); ArrayList〈? extends Alpha〉 list5 = list4; 哪些片段分别插入到第4行,可允许代码编译?()A只有s1B只有s3C只有s1和s3D只有s1和s4
单选题
1. import java.util.*; 2. class SubGen { 3. public static void main(String [] args) { 4. //insert code here 5. } 6. } class Alpha { } class Beta extends Alpha { } class Gamma extends Beta { } 和四段代码片段: s1. ArrayList〈? extends Alpha〉 list1 = new ArrayList〈Gamma〉(); s2. ArrayList〈Alpha〉 list2 = new ArrayList〈? extends Alpha〉(); s3. ArrayList〈? extends Alpha〉 list3 = new ArrayList〈? extends Beta〉(); s4. ArrayList〈? extends Beta〉 list4 = new ArrayList〈Gamma〉(); ArrayList〈? extends Alpha〉 list5 = list4; 哪些片段分别插入到第4行,可允许代码编译?()
A
只有s1
B
只有s3
C
只有s1和s3
D
只有s1和s4
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1. import java.util.*; 2. class SubGen { 3. public static void main(String [] args) { 4. //insert code here 5. } 6. } class Alpha { } class Beta extends Alpha { } class Gamma extends Beta { } 和四段代码片段: s1. ArrayList〈? extends Alpha〉 list1 = new ArrayList〈Gamma〉(); s2. ArrayList〈Alpha〉 list2 = new ArrayList〈? extends Alpha〉(); s3. ArrayList〈? extends Alpha〉 list3 = new ArrayList〈? extends Beta〉(); s4. ArrayList〈? extends Beta〉 list4 = new ArrayList〈Gamma〉(); ArrayList〈? extends Alpha〉 list5 = list4; 哪些片段分别插入到第4行,可允许代码编译?() A、只有s1B、只有s3C、只有s1和s3D、只有s1和s4
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