fix #I2AQ2M

hutool-core/src/main/java/cn/hutool/core/util/RuntimeUtil.java

@@ -298,7 +298,7 @@ public class RuntimeUtil {
 	 *
 	 * @return 最大可用内存
 	 */
-	public final long getUsableMemory() {
+	public static long getUsableMemory() {
 		return getMaxMemory() - getTotalMemory() + getFreeMemory();
 	}
 }

hutool-core/src/test/java/cn/hutool/core/util/RuntimeUtilTest.java

@@ -1,11 +1,10 @@
 package cn.hutool.core.util;
 
+import cn.hutool.core.lang.Console;
+import org.junit.Assert;
 import org.junit.Ignore;
 import org.junit.Test;
 
-import cn.hutool.core.lang.Console;
-import cn.hutool.core.util.RuntimeUtil;
-
 /**
  * 命令行单元测试
  * @author looly
@@ -26,4 +25,9 @@ public class RuntimeUtilTest {
 		String str = RuntimeUtil.execForStr("cmd /c dir");
 		Console.log(str);
 	}
+
+	@Test
+	public void getUsableMemoryTest(){
+		Assert.assertTrue(RuntimeUtil.getUsableMemory() > 0);
+	}
 }