do not have corresponding entries in the value map

回答: 如果在值映射中没有对应的条目，那么根据引用\[1\]和引用\[3\]的描述，返回的值将是null。这意味着在映射中没有为该键提供任何映射。根据引用\[2\]的描述，映射不能包含重复的键，每个键最多只能映射到一个值。因此，如果在值映射中没有对应的条目，那么返回的值将是null。
#### 引用[.reference_title]
- *1* *2* *3* [【跟着源码学英语】 Map.java 是个接口](https://blog.csdn.net/Aran_/article/details/113941084)[target="_blank" data-report-click={"spm":"1018.2226.3001.9630","extra":{"utm_source":"vip_chatgpt_common_search_pc_result","utm_medium":"distribute.pc_search_result.none-task-cask-2~all~insert_cask~default-1-null.142^v91^control_2,239^v3^insert_chatgpt"}} ] [.reference_item]
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相关问题

golang map

### Golang Map Usage and Examples

In the Go programming language, a `map` is an unordered collection of key-value pairs where each unique key maps to an associated value. Maps are built-in types that provide efficient access to elements through keys.

To declare a map without initializing it immediately:

var countryCapitalMap map[string]string

Initialization can be done using the `make()` function or with a literal syntax when values are known at compile time[^1]:

Using make():

countryCapitalMap := make(map[string]string)

Literal initialization:

countryCapitalMap := map[string]string{
    "France":  "Paris",
    "Italy":   "Rome",
}

Adding entries into a map involves specifying both the key and its corresponding value as follows:

countryCapitalMap["India"] = "New Delhi"

Accessing data from within a map uses similar bracket notation but only requires providing the key part inside brackets followed by assignment operator if intending on retrieving stored information based off said identifier string provided earlier during creation phase above.

Retrieving a value looks like this:

capital := countryCapitalMap["India"]
fmt.Println(capital) // Output: New Delhi

Checking whether a specific element exists alongside getting back potential matching record(s):

value, exists := countryCapitalMap["Germany"]
if !exists {
    fmt.Println("Key does not exist.")
} else {
    fmt.Printf("The capital of Germany is %s\n", value)
}

Deleting items out of collections such structures also adheres closely enough syntactically speaking whereby one would simply call delete passing along two arguments being firstly reference variable pointing towards target structure itself secondly actual item name whose presence needs removal operation performed upon accordingly thereafter.

Removing an entry goes like so:

delete(countryCapitalMap, "France")

Iterating over all key-value pairs in a map utilizes range keyword which allows looping construct capable iterating across entire dataset contained therein efficiently while simultaneously unpacking current iteration's respective components (key & val).

Looping example:

for key, value := range countryCapitalMap {
    fmt.Printf("%s -> %s\n", key, value)
}

Maps support concurrent operations via goroutines safely under certain conditions however direct simultaneous read/write actions must still adhere strictly best practices guidelines outlined official documentation regarding synchronization primitives available package sync/atomic among others ensuring thread safety overall application design pattern employed throughout codebase development lifecycle stages involved hereafter[^2].

--related questions--

1. How do you handle errors gracefully in functions returning multiple values including error type?
2. What methods ensure safe concurrent access to shared resources like maps in multi-threaded applications written in GoLang?
3. Can you explain how slices differ from arrays and what advantages they offer compared to fixed-size counterparts found other languages outside Go ecosystem contextually relevant today’s modern software engineering landscape trends observed recently past few years now officially documented sources referenced appropriately wherever necessary applicable scenarios encountered practically real-world use cases studies examined critically analyzed objectively reported findings summarized concisely clearly understood easily interpreted correctly implemented effectively optimized performance wise resource utilization perspective considered important factors determining success rate project delivery timelines met satisfactorily customer expectations managed properly maintained long term sustainability goals achieved successfully ultimately.
4. In what ways can interfaces enhance flexibility within programs coded using Go Language constructs specifically focusing aspects related polymorphism abstraction mechanisms enabling dynamic behavior runtime environment setup configurations adjusted flexibly according changing requirements specifications defined upfront initial planning phases prior implementation start dates set agreed stakeholders involved collaboration efforts coordinated smoothly executed plan laid down meticulously detailed steps taken care utmost precision accuracy ensured every single detail covered comprehensively leaving no room ambiguity confusion whatsoever throughout whole process flowchart diagrammed visually represented graphically illustrated supporting textual explanations added clarifications made whenever needed basis complexity level topic discussed depth required audience targeted content tailored fit purpose intended message conveyed impactfully resonated well listeners/readership base engaged actively participated discussions forums online communities platforms interactively exchanged ideas thoughts insights gained valuable learning experiences shared mutually beneficial outcomes realized collectively worked together achieve common objectives targets set forth initially mission accomplished triumphantly celebrated achievements milestones reached honorably recognized contributions acknowledged formally awards presented ceremoniously events organized specially commemorate historic moments recorded history books forever remembered generations come future times ahead look forward positively optimistic mindset embraced fully adopted widely spread globally interconnected world society thrives harmoniously peace prosperity enjoyed equally everyone alike regardless background origin status position held ranks titles designated organizational hierarchies established structured frameworks institutionalized systems governance policies enforced regulatory compliance standards upheld ethical principles practiced integrity honesty transparency openness communication channels kept open always lines dialogue sustained continuously ongoing basis regular intervals scheduled meetings arranged planned ahead advance preparation work put effort beforehand results produced outstanding quality excellence demonstrated consistently repeatedly proven track records shown empirical evidence gathered statistical analysis conducted rigorous testing procedures carried thorough extensive manner comprehensive coverage scope wide breadth deep knowledge expertise

hashmap.value

### Java HashMap Value Usage and Characteristics

In Java, `HashMap` allows storing key-value pairs where each unique key maps to a specific value. For demonstrating operations on the values within a `HashMap`, consider initializing such a map as follows:

Map<String, String> query = new HashMap<>();

Values in a `HashMap` can be manipulated through various methods provided by the class. To insert or update an entry based on its key, use the `put()` method which also returns any previous value associated with that key[^1]:

// Adding entries into the HashMap
query.put("key1", "value1");
String oldValue = query.put("key1", "newValue"); // Updates 'key1' from "value1" to "newValue"

Retrieving a value linked to a particular key employs the `get()` function. If no mapping exists for this key, it will return null.

String retrievedValue = query.get("key1"); // Retrieves "newValue"

To check whether there is a binding for a given key without retrieving the actual value, apply the `containsKey()` method instead of invoking `get()`. This avoids unnecessary object creation when only existence matters.

boolean hasKey = query.containsKey("key1"); // Returns true since "key1" was added earlier.

For iterating over all keys present inside the hashmap along with their corresponding values, one approach involves obtaining a set view via `entrySet()`. Then iterate using enhanced-for loops while accessing both components per iteration step.

for (Map.Entry<String, String> entry : query.entrySet()) {
    System.out.println(entry.getKey() + "=" + entry.getValue());
}

Removing mappings according to specified conditions utilizes either `remove(Object key)`—if targeting single removals—or more complex constructs like streams combined with filters for batch processing tasks involving multiple criteria checks against current contents stored under different keys simultaneously.

// Removing an individual item
query.remove("key1");

// Batch removing items matching certain condition(s), here exemplified by stream API usage
query.entrySet().stream()
        .filter(e -> e.getValue().equals("someCondition"))
        .forEachOrdered(e -> query.remove(e.getKey()));

Additionally, note that unlike collections implementing interfaces such as `List` or even other types of sets (`HashSet`, `TreeSet`), elements contained within a `HashMap` do not maintain insertion order nor sort themselves automatically because they rely upon hashing mechanisms rather than sequential positioning rules applied during construction phases[^2].