13-16几乎是java语法讲解😄
Primitives Cannot Be Used as Actual Type Arguments
Autoboxing
Wrapper Types Are (Mostly) Just Like Any Class
8种基本类型之间转换也存在widening
Immutability
类似于const在cpp
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public class Date {
public final int month;
public final int day;
public final int year;
private boolean contrived = true;
public Date(int m, int d, int y) {
month = m; day = d; year = y;
}
}
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Warning: Declaring a reference as Final does not make object immutable.
- Example: public final ArrayDeque d = new ArrayDeque();
- The d variable can never change, but the referenced deque can!
见指针常量与常量指针的区别in C++
Defining Generic Classes
Goal 1: Create a class ArrayMap with the following methods:
- put(key, value): Associate key with value. If -1, adds k and v to the last position of the arrays.
- containsKey(key): Checks to see if arraymap contains the key.
- get(key): Returns value, assuming key exists..
- keys(): Returns a list of all keys.
- size(): Returns number of keys.
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public class ArrayMap<K, V> {
private K[] keys;
private V[] values;
private int size;
public ArrayMap() {
keys = (K[]) new Object[100];
values = (V[]) new Object[100];
size = 0;
}
private int findKey(K key) {
for (int i = 0; i < size; i++) {
if (keys[i].equals(key)) {
return i;
}
}
return -1;
}
private int getKeyIndex(K key) {
for (int i = 0; i < size; i++) {
if (keys[i].equals(key)) {
return i;
}
return -1;
}
}
public void put(K key, V value) {
int i = getKeyIndex(key);
if (i > -1) {
values[i] = value; return; }
keys[size] = key;
values[size] = value;
size += 1;
}
public V get(K key) {
return values[findKey(key)];
}
public boolean containsKey(K key) {
int i = findKey(key);
return (i > -1);
}
public List<K> keys() {
List<K> list = new ArrayList<K>();
for (int i = 0; i < size; i++) {
list.add(keys[i]);
}
return list;
}
public int size() {
return size;
}
}
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Generic Methods
Goal: Create a class MapHelper with two methods:
- get(Map61B, key): Returns the value corresponding to the given key in the map if it exists, otherwise null.
Unlike the ArrayMap’s get method, which crashes if the key doesn’t exist.
- maxKey(Map61B): Returns the maximum of all keys in the given ArrayMap. Works only if keys can be compared.
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public class MapHelper {
public static <X,Z> Z get(ArrayMap<X,Z> map, X key) {
if (map.containsKey(key)) {
return map.get(key);
} else {
return null;
}
}
public static <X extends Comparable<X>,Z> X maxKey(ArrayMap<X,Z> map) {
X max = null;
for (X key : map.keys()) {
if (max == null || key.compareTo(max) > 0) {
max = key;
}
}
return max;
}
}
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Wrapper Types Are (Mostly) Just Like Any Class