How to restrict type parameters in Java generic methods?

<p>In order to restrict type parameters in Java generic methods, you need to use the <T extends Bound> syntax, where Bound is a type or interface. As such, parameters only accept types that inherit from the Bound type or implement the Bound interface. For example, <T extends Comparable<T>> restricts T to types that can be compared with itself. </p> <p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/887/227/171445500668359.jpg" class="lazy" alt="如何在 Java 泛型方法中限制类型参数？"></p> <p><strong>How to restrict type parameters in Java generic methods</strong></p> <p><strong>Preface</strong></p> <p> Java generics enable you to create reusable code regardless of the actual type passed to the code. Sometimes, it is necessary to restrict generic type parameters to ensure that the code behaves the expected way. </p> <p><strong>Bounds usage</strong></p> <p>Use the <code><</code> and <code>extends</code> qualifiers to limit generic type parameters. By specifying a type or interface, you can restrict parameters to only accept types that inherit from that type or implement that interface. </p> <p>Syntax: </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>public <T extends Bound> void someMethod(T arg) { // ... }</pre><div class="contentsignin">Copy after login</div></div><p><strong>Practical case</strong></p><p>Consider a <code>compare</code> method that compares the size of two elements: </p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class='brush:php;toolbar:false;'>public <T extends Comparable<T>> void compare(T a, T b) { if (a.compareTo(b) > 0) { System.out.println("a is greater than b"); } else if (a.compareTo(b) < 0) { System.out.println("a is less than b"); } else { System.out.println("a is equal to b"); } }</pre><div class="contentsignin">Copy after login</div></div> <p>Here, a parameter of type <code>T</code> is constrained to implement the <code>Comparable</code> interface, which means it can be compared with itself. </p> <p><strong>Other limits</strong></p> <ul> <li> <strong>Multiple limits: </strong>You can specify multiple limits using the <code>&</code> operator. For example, <code><T extends Comparable<T> & Serializable></code>. </li> <li> <strong> Native types: </strong> Native types can be restricted by using wrapper classes. For example, <code><T extends Number></code>. </li> <li> <strong>Wildcard: </strong>You can use the <code>?</code> wildcard to further restrict the type without specifying a specific type. For example, <code><T <? extends Number></code>. </li> </ul> <p><strong>Notes</strong></p> <ul> <li> Ensure that type parameters meet the specified boundaries, otherwise compilation errors may occur. </li> <li>Restricting type parameters helps ensure code robustness and maintainability. </li> </ul> <p><strong>Additional Resources</strong></p> <ul><li>[Official Java Documentation: Generics](https://docs.oracle.com/javase/tutorial/java /generics/)</li></ul>

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