1. Switch expression
In previous releases, switch expression was only a feature in the "preview" stage. I would like to remind you that the purpose of the features in the "preview" stage is to collect feedback. These features may change at any time, and based on the feedback results, these features may even be removed, but usually all preview features will be fixed in Java in the end. .
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The advantage of the new switch expression is that there is no longer a default skip behavior (fall-through), and more Comprehensive, and expressions and combinations are easier to write, so bugs are less likely to occur. For example, switch expressions can now use arrow syntax, as shown below:
var log = switch (event) {
case PLAY -> "User has triggered the play button";
case STOP, PAUSE -> "User needs a break";
default -> {
String message = event.toString();
LocalDateTime now = LocalDateTime.now();
yield "Unknown event " + message +
" logged on " + now;
}
};2. Text Blocks
One of the preview features introduced in Java 13 is text blocks. With text blocks, multi-line string literals are easy to write. This feature is getting its second preview in Java 14, and there are some changes. For example, formatting of multiline text may require writing many string concatenation operations and escape sequences. The following code demonstrates an HTML example:
String html = "<HTML>" + "\n\t" + "<BODY>" + "\n\t\t" + "<H1>\"Java 14 is here!\"</H1>" + "\n\t" + "</BODY>" + "\n" + "</HTML>";
With text blocks, this process can be simplified. Just use triple quotes as the start and end tags of the text block, and you can write more elegant Code:
String html = """ <HTML> <BODY> <H1>"Java 14 is here!"</H1> </BODY> </HTML>""";
Text blocks are more expressive than ordinary string literals.
Java 14 introduces two new escape sequences. First, you can use the new \s escape sequence to represent a space. Second, you can use the backslash \ to avoid inserting a newline character at the end of the line. This makes it easy to break up a long line into multiple lines within a block of text to increase readability.
For example, the way to write a multi-line string now is as follows:
String literal = "Lorem ipsum dolor sit amet, consectetur adipiscing " + "elit, sed do eiusmod tempor incididunt ut labore " + "et dolore magna aliqua.";
Using the \ escape sequence in a text block, it can be written like this:
String text = """ Lorem ipsum dolor sit amet, consectetur adipiscing \ elit, sed do eiusmod tempor incididunt ut labore \ et dolore magna aliqua.\ """;
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3. Pattern matching of instanceof
Java 14 introduces a preview feature, with which it is no longer You need to write code that first passes instanceof judgment and then forced conversion. For example, the following code:
if (obj instanceof Group) {
Group group = (Group) obj;
// use group specific methods
var entries = group.getEntries();
}Using this preview feature, it can be refactored into:
if (obj instanceof Group group) {
var entries = group.getEntries();
}Since the conditional check requires obj to be of Group type, why do we need to include it in the conditional code like the first piece of code? What about specifying obj as Group type in the block? This may cause errors.
This more concise syntax can eliminate most casts in Java programs.
JEP 305 explains this change and gives an example from Joshua Bloch's book "Effective Java", demonstrating the following two equivalent ways of writing:
@Override public boolean equals(Object o) {
return (o instanceof CaseInsensitiveString) &&
((CaseInsensitiveString) o).s.equalsIgnoreCase(s);
}This paragraph The redundant CaseInsensitiveString cast in the code can be removed and transformed into the following way:
@Override public boolean equals(Object o) {
return (o instanceof CaseInsensitiveString cis) &&
cis.s.equalsIgnoreCase(s);
}This preview feature is worth trying because it opens the door to more general pattern matching. The idea of ??pattern matching is to provide a convenient syntax for the language to extract components from objects based on specific conditions. This is exactly the use case of the instanceof operator, because the condition is a type check, and the extraction operation requires calling the appropriate method, or accessing a specific field.
In other words, this preview function is just the beginning. In the future, this function will definitely reduce more code redundancy, thereby reducing the possibility of bugs.
4. Record
Another preview function is record. Like other preview functions introduced earlier, this preview function also follows the trend of reducing redundant Java code and can help developers write more accurate code. Record is mainly used for classes in specific fields. Its displacement function is to store data without any custom behavior.
Let’s get straight to the point and take the simplest example of a field class: BankTransaction, which represents a transaction and contains three fields: date, amount, and description. There are many aspects to consider when defining a class:
Constructor, getter, method toString(), hashCode() and equals(). The code for these parts is usually automatically generated by the IDE and takes up a lot of space. Here is the complete generated BankTransaction class:
public class BankTransaction {private final LocalDate date;
private final double amount;
private final String description;
public BankTransaction(final LocalDate date,
final double amount,
final String description) {
this.date = date;
this.amount = amount;
this.description = description;
}
public LocalDate date() {
return date;
}
public double amount() {
return amount;
}
public String description() {
return description;
}
@Override
public String toString() {
return "BankTransaction{" +
"date=" + date +
", amount=" + amount +
", description='" + description + '\'' +
'}';
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
BankTransaction that = (BankTransaction) o;
return Double.compare(that.amount, amount) == 0 &&
date.equals(that.date) &&
description.equals(that.description);
}
@Override
public int hashCode() {
return Objects.hash(date, amount, description);
}
}Java 14 provides a way to resolve this redundancy and express the purpose more clearly: the only purpose of this class is to bring data together. Record will provide implementations of equals, hashCode and toString methods. Therefore, the BankTransaction class can be refactored as follows:
public record BankTransaction(LocalDate date,double amount,
String description) {}Through record, you can "automatically" get the implementation of equals, hashCode and toString, as well as the constructor and getter methods.
To try this example, you need to compile the file with the preview flag:
javac --enable-preview --release 14 The fields of BankTransaction.javarecord are implicitly final. Therefore, record fields cannot be reassigned. But it should be noted that this does not mean that the entire record is immutable. The objects stored in the fields can be mutable.
5. NullPointerException
一些人認(rèn)為，拋出NullPointerException異常應(yīng)該當(dāng)做新的“Hello World”程序來看待，因為NullPointerException是早晚會遇到的。玩笑歸玩笑，這個異常的確會造成困擾，因為它經(jīng)常出現(xiàn)在生產(chǎn)環(huán)境的日志中，會導(dǎo)致調(diào)試非常困難，因為它并不會顯示原始的代碼。例如，如下代碼：
var name = user.getLocation().getCity().getName();
在Java 14之前，你可能會得到如下的錯誤：
Exception in thread "main" java.lang.NullPointerExceptionat NullPointerExample.main(NullPointerExample.java:5)
不幸的是，如果在第5行是一個包含了多個方法調(diào)用的賦值語句（如getLocation()和getCity()），那么任何一個都可能會返回null。實際上，變量user也可能是null。因此，無法判斷是誰導(dǎo)致了NullPointerException。
在Java 14中，新的JVM特性可以顯示更詳細(xì)的診斷信息：
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "Location.getCity()" because the return value of "User.getLocation()" is nullat NullPointerExample.main(NullPointerExample.java:5)
該消息包含兩個明確的組成部分：
后果：Location.getCity()無法被調(diào)用原因：User.getLocation()的返回值為null增強(qiáng)版本的診斷信息只有在使用下述標(biāo)志運(yùn)行Java時才有效：
-XX:+ShowCodeDetailsInExceptionMessages
下面是個例子：
java -XX:+ShowCodeDetailsInExceptionMessages NullPointerExample
在以后的版本中，該選項可能會成為默認(rèn)。
這項改進(jìn)不僅對于方法調(diào)用有效，其他可能會導(dǎo)致NullPointerException的地方也有效，包括字段訪問、數(shù)組訪問、賦值等。
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